Man, a top twenty list... Oh well, I can try at least.
FROM BEST TO... WELL, NOT BEST, BUT STILL GREAT!
1: Banjo Kazooie. (I won't deny the nostalgia, but I also love the humor, which is definitely the strongest part of the game.)
2: Dark Souls. (Love it, and I really don't understand why people find it hard... I never did.)
3: Hitman: Blood Money. (Holy shit, an assassination game where you AREN'T insanely OP *CoughAssassinsCreedCough*)
4: STALKER: Shadows of Chernobyl. (For me, atmosphere is the most important part, and this game has it in spades.)
5: Half-Life 2: Episode 2. (Really the whole franchise, but mostly 2 because it has the best atmosphere.)
6: Shadows of the colossus. (Atmosphere~)
7: Legend of Zelda: Wind Waker. (Love the world, the characters, and the atmosphere... The sailing's fun too.)
8: Devil May Cry 3: Special Edition. (I absolutely love Hack'n'slash games)
9: Tekken 6. (Purely because of Dragunov. God, is he fun to use.)
10: Bastion. (It's just awesome. Play it, now, I DEMAND IT!)
11: Pokemon Gen 4. (I just like that generation best. That, and it has probably the most evil villian.)
12: Jade Cocoon 2. (Only played it once, but damn did it leave a lasting impression.)
13: The Binding Of Isaac. (When it doesn't fuck me over that is.)
14: Dragon Age: Origins. (Awesome world, awesome story, awesome characters, piss-poor fighting system.)
15: Legend of Grimrock. (I love dungeon crawlers.)
16: Diablo 2. (My very first dungeon crawler, and by god do I love it.)
17: Heroes of might and magic 6. (Dunno why, just love it.)
18: Recettear: An item shop's tale. (You have no idea how surprised I was when I found out that I love this game.)
19: Prototype. (Extremely cathartic.)
20: Super Mario World. (First game I remember playing, so it has a special place in my heart.)
And this is my list. Looking over it, I can honestly say I am surprised by the variety.