I don't think I can list them in order, sorry. I'm not sure I can pick one game out of some franchise either. Again, sorry!
Sonic 1 and Sonic 2 (The games that got me in to gaming)
Mickey and Donald: World of Illusion (I'm not a Disney fan by a long shot, but this is one of the best games ever made)
Grim Fandango (What's not to love? Some of the best characters ever made, great plot, great style, great puzzles and funny)
Monkey Island 1, 2 and 3 (Again, funny, great puzzles, well written and some cool characters)
Metal Gear Solid (The whole series is amazing, 4's game play wasn't the best, but who can not love MGS?)
Half-Life (The original got me in to FPSs and the 2nd was just a groundbreaking masterpiece!)
Final Fantasy I- IX (I love everything about final fantasy from 1 to 9, shame they're a pile of turd since X though)
Max Payne 1 and 2 (Such cool games. 3 is alright, but not worthy of best game ever status)
Maplestory (Feel like I may get flamed for this but ,eh? It was one of the best MMO's ever back in the day)
Bioshock (Incredible setting and story)
Fallout 3 (Just loved the game so much, great setting, great music, cool concept and three-dog 'hooowl')
Legend of Zelda: Ocarina of Time (No need for explanation)
Resident Evil 2 (The best horror game ever made, the first time I've ever been scared to play a game. new pants please!)
Counter Strike: Source (The best tactical multiplayer FPS ever made. I have over 4000 hours played, ha ha)
Tony Hawk's Pro Skater 1-3 (So fun to play back as a kid, got me in to skateboarding too)
Grand Theft Auto: Vice City (Everyone knows why)
Braid (Such a fantastic story, some nice puzzles and quite possibly the best game ending ever made)
Terraria (Such a neat little game, great visuals, great exploration and fun as heck on multiplayer)
Street Fighter 2 (Best beat 'em up ever made, you all agree I'm sure)
Tomb Raider 1 and 2 (Guns, Dinosaurs and massive tits. What more did a boy need?)