Caveman to Cosmos is such a big mod that I''m pretty sure the game engine collapses most of the time trying to run it.

Fallout New Vegas: JSawyer mod Ultimate Edition

Skyrim: Sexlab. Then I get down to actually playing the game and remember all the reasons why I uninstalled it last time before getting to lvl10. I'd much rather deal with FNV's technical jank than Bethesda's cardboard-flavored writing.

Diablo - The Hell 1 HD
Everyday I'm glad that I grabbed this mod before it was taken down.
Technical improvements (resolution, 60FPS, windowed) for TH1 is all you need, TH2 change way too much in gameplay department.

Temple +
For Temple of Elemental Evil, mandatory these days and way better than old Circle of Eight.
Funny that all Troika games need patches/mods to play but then you get unique games that no indie or other developer delivered so far.

Ballistic Weapons
for UT2004, silly but fun arsenal with gameplay additions make good relaxing game

HD mods :

Final Fantasy VII Satsuki's mod, it crushes the "remako" attempt to bits.

Final Fantasy IX Moguri's mod

Gameplay & Hd :

Morrowind MOSG (overhaul)

I only play Skyrim, and my favorite mod is Hunterborn.

Accompanied by about 150 other mods, many about crafting. Equally important are those that help me avoid quests. I just wander around, craft, and buy houses, every time.

Not a knock, but I was wondering when a Sex mod would come up.

Torchlight: Darker Textures by SorrySouldOut (Hate the cartoon style).

Skyrim: there's so many of them... uhm... Unofficial Patch, sse and SkyUI, but I guess everyone has them :D. Static Mesh Improvement, sounds of skyrim, A Quality World Map, Cutting Room Floor, winter is cmming and cloaks of Skyrim (must have), 2k textures, Realistic Lighting Overhaul. Realistic Water Two, immersive HUD. Right now I'm playing Enderal: Forgotten Stories (TC).

Descent: D2X-XL and DXX-Rebirth.

Fallout 1 and 2: Fixt and Restoration Project.

Fallout 3: enabling support for 4GB LAA, Multicore and Threading, Wanderer's Edition, Mart's Mutant Mod, NMC's Texture Pack, Fellout, Enhanced Weather. Gopher's screenfilter mod. No ENB. Unofficial Fallout 3 Patch.

And lastly GZdoom, Brutal Doom and Brutal Hexen.