Aside from Max Payne and Just Cause 2, i havent really tried any mods with my PC games. 
 
Which games (exclusive or multiplatform) have the best mods on the PC ? not just ammo or health changes, but really game-changing stuff 
 
Edit: could you tell us what mods do ?

Crysis, or crysis warhead. I don't use the mods and stuff, however there is a ton of things you can do and even extra levels to download.

come on, guys ! 
  
are there no games with great Mods on the PC ?

TES games usually.

Oblivion and fallout 3 have tons of mods.

Oblivion, Crysis is amazing. Fallout 3 and Vegas. Mass Effect. Dragon Age  And every Infinity Engine game 

Bethesda games (Oblivion, Fallout), Mount and Blade, Total War series, Half Life series, Crysis series. Dragon Age can be modded a fair amount as well. 

Source Engine games have tons of mods, go on moddb if you are really apt to finding some good ones.

Most RTS games tend to have huge mod scenes. I mean, just look at Medieval 2 Total War. You could go from 9th century Anglo-Saxon England to Middle Earth. Mount & Blade and Mount & Blade: Warband also have huge developer supported mod scenes.

"  Crysis is amazing    "

" Half Life series "

" Source Engine games have tons of mods, go on moddb if you are really apt to finding some good ones. "

Bethesda games have a ton of mods, mostly because they've been using the same ass engine for like a decade so basically a new game comes out, and every mod from the previous game is quickly transferred. Mart's Mutant Mod, specifically, makes Fallout 3 actually fun.

"  @5socks5 said: 

"  Crysis is amazing    "

what kind of mods are there for Crysis? what do they do ? anything specific ? 
 
@Geno said: 

" Half Life series "

@IcySandman said: 

" Source Engine games have tons of mods, go on moddb if you are really apt to finding some good ones. "

are we talking funky Garry's Mod stuff or are there really good gameplay-related mods? "

" @5socks5 said:

"  Crysis is amazing    "

what kind of mods are there for Crysis? what do they do ? anything specific ? 
 
@Geno said:

" Half Life series "

@IcySandman said:

" Source Engine games have tons of mods, go on moddb if you are really apt to finding some good ones. "

are we talking funky Garry's Mod stuff or are there really good gameplay-related mods? "

All types of mods for Crysis. Vanilla Crysis has lots of issues including low-res textures, poor lighting and time of day, lack of anisotrophic filtering, and overly aggressive LoD. There are graphical mods that fix all that. There are also level mods, suit and weapon mods, and other good stuff.  
 
For Source engine there's tons and tons of mods for it (nothing to do with Garry's Mod); I haven't followed Source for a while but ModDB will hook you up. 

my favorite mods were multiplayer mods from the Quake 2 and the UT2004 days.
There are so many mods out there it's hard to keep up. (moddb.com)
Now i only add mods if i think a game needs to be fixed (STALKER).

GTAIV also has some nice mods, but last I heard they were a bit tricky to work with. 

Half life 1 and 2 have a TON of mods, multiplayer and single player  
Go check out Moddb.com 

Check out some of the total conversion mods for Half Life 2 like Minerva.

Morrowind. Still has an active community, and people are working on making the engine open. So it will only get better. 
 
 
Also, DOOM.

Neotokyo is a source engine mod, I played it when it first began, IDK what it's like now but it was pretty fun.  Yeah moddb is a website that alot of modders go to/people that want to download new mods.  Stalker SOC and the other ones have some cool mods.

I hear Doom 3 has a number of mods of the basically-creating-an-original game type.

STALKER: Shadow of Chernobyl has mods that literally alter the game completely to becoming better both visually and gameplay-ishly.

I have a kick ass revolutionary war mod for Half Life 2 you should check out, i know you want to!  
 
And I have a fallout 3 folder filled with... umm... skins.  Mass Effect 2 has some text mods, I enjoyed my fully automatic sniper rifle with 2000 damage.