Surely listing 20 is too much. Noone is going to read that much text from so many posters. Top 5 would have been better, but as an additional protest, I'll list 4:

4. Ultima VII
3. Fallout 2
2. Deus Ex
1. Jagged Alliance 2

Couldn't think of enough RPG's. Anyway, here is the list in alphabetical order.

Age of Empires 2
Battlefield BC 1/2 (mp)
Chrono Trigger
Dead Space
Demon's Soul
Eve Online
GTA Vice City
Mafia
Mario Golf: Advance Tour
Monkey Island 1-2
Maniac Mansion
NHL 94
PES 6
RDR
Sonic 2
The Legend of Zelda: The Wind Waker
Tie Fighter
Tropico
Ultima 5

Should probably thrown in a racing game as well, but can't decide which.
So one NFS/PGR game.

edit. maybe the first NFS Hot Pursuit then

Yeesh! This was more difficult than I thought it would be... I put them in alphabetical order because trying to rank them would be an impossible task. All are PC versions, and always assume the most complete version of the game (including expansions, DLC, etc.) unless specifically stated otherwise.

Baldur's Gate
Baldur's Gate II
Batman: Arkham City
Commander Keen: Episode 4 - Secret of the Oracle
Crusader: No Remorse
Diablo
DOOM
DOOM II: Hell on Earth
Fallout
Fallout 2
Heroes of Might and Magic III
Icewind Dale
Icewind Dale II
King's Bounty: Armored Princess
Little Big Adventure 2: Twinsen's Odyssey
Mass Effect
Mass Effect 2 with Lair of the Shadow Broker*
Neverwinter Nights 2: Mask of the Betrayer and Storm of Zehir**
Planescape: Torment
Unreal


* This was the only DLC that interested me. I never purchased or played any of the others.

** I love these expansions. They were such vast improvements over the original NWN2 campaign in every way, that they deserve their own spot on this list. Even if the core game didn't make it on here.

Well better than me good sir. Gaming since a kid and still suck at it. Thankfully, they take people like me into consideration and add "easy mode".

On another note aimed toward the whole forum, do you really think so highly of Doom 2? Honestly, playing through it seemed not at the caliber I remembered it to be. Particularly the awful map design (Only ones that seemed fun were Romero's and McGee's) along with nothing really substantial save for the double-barrel shotgun. Maybe I just hate the Pain Element and Archevil so much that I'm taking it out on the game as a whole.

Well, when putting together my list, I chose to pick the games that had the biggest and longest lasting impact on me as a gamer and that I remembered most fondly. I was 13-14 years old when DOOM and DOOM II were released and had never seen anything like them... I never really played DOOM II as a traditional sequel -- meaning after finishing the original (or at least playing it for an extended period of time) -- but instead, I pretty much played both games alongside each other, switching back and forth between them, trying different maps, etc. and I don't believe I was too knowledgeable about things like map design at the time (or had anything to compare it to, really), and I didn't care... I was having a blast.

So that's why they're on my list, at least :-) they made a huge impact on me, and a major contribution to my love of gaming. And nostalgia allows me to still play and enjoy them today.

i think doom 1+2 still kick ass
the graphics dont hold up so well today, i am saying that as someone who still considers stuff like mgs1 or final fantasy 6 as games that still hold up graphically, the design is timeless, and they dont hurt your eyes technical wise, but doom 1+2 i have difficulties to look at, apart from that those are still two great games, and for graphics there are mods out there which make them smoother and easier to look at

It's all a matter of subjective opinion. You have some great games on your list, but Minecraft? How can you people look at this and do not get headaches? For gods sake, it's 2012! "Models" and textures in this game looks unbelievably crappy. I was interested in the concept but I just can't swallow the implementation. Games 10 or 15 years old look better... But no offence mate, it's just my opinion :)