Tip: Scroll through the images above with right and left arrows or space and backspace. Opening the three images in new tabs will give you a better indication of quality difference.
More comparisons can be found in the downloads.

Please don't take any screenshot as full demonstration of all textures in these mods. Cannot cover every texture applied by mods with in-game screenshots.
STEP - Skyrim Total Enhancement Project is a guide that can be used to customize your Skyrim.
STEP - Skyrim Total Enhancement Project by TheCompiler.

Table of Contents:
1.* Texture Mods and one Mesh mod to rule them all
2.* Benchmarking Texture Mods
3.* Creating your own combination of textures
4.* DDS (texture) Stats for versions specified
5.* Credits
* What is DDS? Microsoft DirectDraw Surface (.dds) texture file format. That's the texture files these mods provide.

1.- Texture Mods and one Mesh mod to rule them all:
-BETH_HR; High Resolution Texture Pack (Free DLC). Improved vanilla textures, some more than others. Have a look at Snoopey0's Comparison Video on YouTube. You may still want to use HD Textures DLC Fix. Had debated whether the DLC really makes a difference. Well it does. Armour, weapons, clothes, sky, effects... You would be very hard pressed to find a combination of mods that replace all of those.
-SMIM; Static Mesh Improvement Mod v1.19. Changes the 3D Mesh (model) of objects like barrels to have a more appealing smooth quality than vanilla. A must have. It has its own textures, some (10) of which may be replaced by texture mods to give new look. The wood posts as seen in the Riverwood screenshot (download) are interesting... a few options:
1. Vanilla Wood post with Vanilla Mesh (3D model)
2. SMIM Wood post with SMIM Mesh
3. HD2K Wood post with SMIM Mesh
4. Realistic Overhaul Wood post with SMIM Mesh... not so different to SMIM Wood post as the texture is not so different, yet the texture quality is better.
-SERIOUS_1024/2048; Serious HD Retexture v1.2.3. Changes Landscape textures, such as grass and snow. Found that many textures are common with HD-2K and Realistic Overhaul.
-HD2K_Lite/Full; HD - 2K Textures v1.5. Many HD Textures. Many common. The textures don't resemble the originals, HD2K doesn't try to. The textures change the style, which may be appealing to those that use them. These textures are well done and of great quality.
-SRO_1024/2048; Realistic Overhaul v1.2. Many HD Textures. Many common. Opposite to HD2K, this one attempts to stay close to original style. Overall, it replaces existing textures with higher quality versions while changing other aspects, like removing stones in unpaved roads (Riverwood). I think I'm going to use this one.
-CHRIS2012s_WR_1024/2048; Chris2012s Whiterun HQ Texture Pack v1.01. Mostly Whiterun and interior textures in other zones.

2.- Benchmarking Texture Mods:
Its the resolution of the textures and how many objects are displayed with them that can impact your Skyrim performance. There's nothing a texture mod can do to help this, other than lowering the resolution and/or quantity of textures. Provided are three benchmarks. SMIM-BETH_HR, MIX_1024-SMIM-BETH_HR and MIX_2048-SMIM-BETH_HR. The MIX will provide the most modded textures, coming in at around 1,400. The texture mods tested my PC combined and shows the impact texture resolution has on my Skyrim performance.
Skyrim Settings:
High (primarily default. multithreading enabled, AA 8x, FXAA Off); See the System_Configurator image for more details.
Test PC:
O/S: Windows 7 64-bit
CPU: Q6600 @ 3.2GHz OC (Intel Quad 2.4GHz)
GPU: ATI Radeon HD 6870 1GB (Catalyst 12.3)
RAM: 4x1GB Kingston DDR2-1066 (5-5-5-15 @ 2.4v)
M/B: Gigabyte X38-DS4
HDD: 1TB Seagate Baracuda 7200.12 (ST31000528AS). Running Win7, Skyrim and textures from this HDD.
And: It's an older PC, nothing special
Benchmark Method: Fraps v3.4.7 (MinMaxAvg) in Whiterun. Planned two minute sneak from market/stalls to Dragonsreach and around. Noticed the skeleton in the moat. Used a similar method as this.
* Between benchmarks, mods were uninstalled and new ones installed. The PC was then restarted and rested for at least 2 minutes before launching Skyrim.
Screenshot Method: Whiterun and Riverwood pre-saves.
Benchmark results:
--- This first test provided for comparison ---
1-SMIM-BETH_HR: FPS; Min 24, Max 62, Avg 57.075
+ Run 2: FPS; Min 2, Max 62, Avg 57.458
--- These test the performance between 1024 and 2048+4096 textures ---
2-MIX_1024-SMIM-BETH_HR: FPS; Min 18, Max 62, Avg 55.192
+ Run 2: FPS; Min 35, Max 62, Avg 57.892
(SMIM SERIOUS_1024 CHRIS2012s_WR_1024 SRO_1024 HD2K_Lite)
3-MIX_2048-SMIM-BETH_HR: FPS; Min 0, Max 62, Avg 45.517
+ Run 2: FPS; Min 0, Max 62, Avg 45.392
(SMIM SERIOUS_2048 CHRIS2012s_WR_2048 SRO_2048 HD2K_Full)
Summary of Results:
Lite and 1024 resolution textures for me. If you've got a newer PC, you might have no stutters and Full and 2048 resolution mods would run fine.
Previous tests showed good overall performance with AA Off and FXAA On. So thats what I'm setting for gaming from now on.
Added DDS files to DATA folder
SMIM ; 61
SMIM SERIOUS_2048 ; 174
SMIM CHRIS2012s_WR_2048 ; 154
SMIM SRO_2048 ; 1,226
SMIM HD2K_Full ; 651
SMIM SERIOUS_1024 CHRIS2012s_WR_1024 SRO_1024 HD2K_Lite ; 1,390 [loading time = 20.5sec]
SMIM SERIOUS_2048 CHRIS2012s_WR_2048 SRO_2048 HD2K_Full ; 1,395 [loading time = 28.2sec]
(mods were installed in order as above. Any existing file was overwritten. SERIOUS and CHRIS2012s_WR would have left little or no contribution of DDS files. I've yet to calculate that. It would be interesting to see if any DDS weren't replaced by SRO and HD2K.)
Update...
SRO (DDS) = 1,166
HD2K (DDS) = 600
SRO -> HD2K
Overwritten/common DDS = 440
SRO Used DDS = 726
HD2K Used DDS = 160
DDS = 1,326
SRO + HD2K -> CHRIS2012s_WR
Overwritten/common DDS = 87
CHRIS2012s_WR Used DDS = 13
DDS = 1,339
SRO + HD2K + CHRIS2012s_WR -> SERIOUS
Overwritten/common DDS = 113
SERIOUS Used DDS = 0
DDS = 1,339
... so no, it was not worth using the SERIOUS. The files were completely overwritten anyway.

3.- Creating your own combination of textures:
You can extract mods manually or you can install multiple texture mods into Nexus Mod Manager (NMM). If you do NMM, make sure to activate them in correct order. Example: SRO_part1, SRO_part2, SRO_part3 and then SRO_update (for SRO v1.2 2048 res update, you will need to manually install, or install with NMM and then go to your data folder and move the textures from the "SRO 1_2 Update/2048 res" folder. To see a more detailed list in NMM, click the " i+ " button on the left after selecting SRO_part from the list and then choose the corresponding mod info. If you leave mod archives as original downloaded name this will be a breeze and the first and only find will re-list your texture mod with part number or update. I select "No", when NMM asks if I want to upgrade the mod.
Doing it this way, you should look into what large texture mod you want primary. It all depends on what textures you keep and what texture you overwrite as to what will be achieved. You will want to install things in opposite order of what textures you want used.
Here's an example, I want to make Realistic Overhaul the primary:
1. Beth HR DLC, activated
2. SMIM, installed and activated (is a Mesh mod. It changes 3D models and has textures too)
3. HD - 2K Textures, activate and overwrite any SMIM texture files (depending on your taste)
4. Realistic Overhaul, activate and overwrite any existing textures to make SRO primary.
With this method will throw together a mix with Realistic Overhaul taking the cake. SRO 1024 v1.2 will overwrite a possible " 440 " HD - 2K Lite textures out of the " 600 " textures HD - 2K Lite provides. HD - 2K Lite would provide " 160 " other textures while SRO 1024 would give " 726 " other textures. That's " 1,326 " new textures all together, excluding any SMIM DDS files.
Compile your own assortment:
Before you attempt this, may I suggest you install a 1024 res and 2048 res texture mod (both of same mod) into Nexus Mod Manager (NMM). Activate SMIM and then one texture mod to test, launch Skyrim and head to Whiterun. Run around a bit, see how your PC performs. Quit the game and deactivate only the texture mod. Test another and so on. At least this will give you some indication. You may have to use lower resolution 1024 res mods.
A good fast way to view *.dds texture files is IrfanView, get the Plugins/AddOns while your there, a plugin provides the DDS view.
Anti-Twin is made for finding and removing duplicate files. Though I find it useful for comparing the extracted texture files by using only the Compare Names option. The preview doesn't support DDS files, though if you have IrfanView, you can open the DDS from Anti-Twin. What does this mean? Well if you extract, for example:
- This will take a lot of time (days probably) and you will have to be careful. Once done though, you will have your very own texture customized Skyrim -
1) To put together the highest resolution combination, extract these to C:\DDS Compare\
a. Skyrim HD - 2K Textures Full, to new folder within DDS Compare
b. Realistic Overhaul 2048K, to new folder within DDS Compare
c. Serious HD Retexture 2048px, to new folder within DDS Compare
d. Chris2012's Whiterun HQ Texture Pack 2048, to new folder within DDS Compare
2) Open Anti-Twin
a. Set the Basic Folder to C:\DDS Compare
b. Click file filter and add "dds" without quotes
c. Deselect Compare content and Select Compare names
d. Set the limit to 50,000 duplicates
e. Start the Search for same names
3) Takes a long time. Compare the textures
a. Mark for deletion those you don't want. Look at the directory structure. Some file names are shared, such as "\textures\dungeons\caves\cavebaseground01_n.dds" and "textures\landscape\cavebaseground01_n.dds". If the image filename has "_n" at the end, it's a Specular shine texture. You will want to keep the corresponding "_n" texture with the same mods similar named texture. See Construction Set Wiki - DDS Files for more information about DDS. Delete when your happy and close Anti-Twin
b. Take the texture files from each mod "texture" folder and combine
c. Use your custom combination with Skyrim.
D. Don't forget the mod authors who created the textures. Endorse them if you use their textures. And don't, DO NOT, upload your customized texture compilation, as doing so would be theft. Use their textures, not rip them off.

4.- DDS (texture) Stats for versions specified:
SMIM v1.19 = 127 total. 10 replaceable via other texture mods.
HD-2K v1.5 Lite = 600
HD-2K v1.5 Full = 600 >
(128 x 128 ; 3) These sorted by height in IrfanView Thumbnails
(256 x 256 ; 3)
(256 x 512 ; 1)
(512 x 512 ; 21)
(1024 x 512 ; 3)
(1024 x 1024 ; 60)
(2048 x 1024 ; 7)
(512 x 2048 ; 1)
(1024 x 2048 ; 15)
(2048 x 2048 ; 448)
(4096 x 2048 ; 9)
(1024 x 4096 ; 4)
(2048 x 4096 ; 5)
(4096 x 4096 ; 20)

Realistic Overhaul v1.2 1024K = 1,166
Realistic Overhaul v1.2 2048K = 1,171 (5 extras over 1024K) >
(1024 x 64 ; 4) These sorted by height in IrfanView Thumbnails
(1024 x 128 ; 2)
(512 x 256 ; 2)
(512 x 512 ; 2)
(1024 x 512 ; 11)
(2048 x 512 ; 2)
(256 x 1024 ; 1)
(512 x 1024 ; 22)
(1024 x 1024 ; 180)
(2048 x 1024 ; 17)
(4096 x 1024 ; 1)
(512 x 2048 ; 2)
(1024 x 2048 ; 19)
(2048 x 2048 ; 900)
(4096 x 2048 ; 2)
(1024 x 4096 ; 2)
(2048 x 4096 ; 2)

Serious HD Retexture v1.2.3 1024px = 113
Serious HD Retexture v1.2.3 2048px = 113 >
(1024 x 1024 ; 2) These sorted by height in IrfanView Thumbnails
(2048 x 2048 ; 111)

Chris2012s Whiterun v1.01 1024 = 100
Chris2012s Whiterun v1.01 2048 = 100 >
(256 x 256 ; 2) These sorted by height in IrfanView Thumbnails
(512 x 512 ; 1)
(1024 x 512 ; 1)
(1024 x 1024 ; 2)
(2048 x 1024 ; 4)
(1024 x 2048 ; 13)
(2048 x 2048 ; 76)
(4096 x 4096 ; 1)

5.- Credits:
Skyrim: Bethesda Softworks
High Resolution Texture Pack (Free DLC): Bethesda Softworks
Static Mesh Improvement Mod - SMIM: Brumbek
Serious HD Retexture Skyrim: Z4G4
Skyrim HD - 2K Textures: NebuLa
Skyrim Realistic Overhaul: Starac
Chris2012s Whiterun HQ Texture Pack: Chris2012