Top 10, sorta, it's hard

01 - City of Heroes / City of Villains
[hider=Why?]It was the most fun I've had in a MMO, ever. The fan-base was great, the game itself was great. Super fun. Love the setting. Love the mission creator. Love the character creator. Love the power customization. Even began to enjoy the super flawed PVP near the end. I really am still, to this day upset that it was shut down. Years have passed, and I still want to log in and play more, and hang out with the friends I made there.[/hider]
02 - Crusader Kings 2
[hider=Why?]Because hegemony through political marriage, war, crusades, and eugenics? That shit is the best. I fucking love this game. I wanna get used to the new patch and do a new AAR. Because ugh, man, I do miss my old AAR and play-through but patches have killed it so I can't do that save. Man this game is good though.[/hider]
03 - Jade Empire
[hider=Why?]It's my most replayed RPG ever. It's the most fun I've had in a non-turn-based RPG ever. It's so good. It's too short. I want more.[/hider]
04 - Sonic 3 & Knuckles
[hider=Why?]Nostalgia incarnate, but good on its own two feet too. 99% of my happy memories from childhood are based around my older brother and I playing this game. This game is so good. Even now, it's so good.[/hider]
05 - Dawn of War 1; Winter Assault & Dark Crusade
[hider=Why?]The most fun I've ever had with a RTS. I played with my friends and against my friends so much, and it was so damned good. Beating AI on the hardest mode the first time in a 3 v 3 match, people vs AI? That was bliss.[/hider]
06 - Persona 4
[hider=Why?]Oh my god this game is so good. It's so fucking good. I just played it so the good is fresh, and maybe later it'll rise up higher, but right now it sits here. A respectable place.[/hider]
07 - Papers, Please
[hider=Why?]I actually feel bad that I forgot this game first time around. This game is perhaps my favourite puzzle game ever. Holy shit. I love the gameplay. I love the story. I love the endings. I love it.[/hider]
08 - Roller-coaster Tycoon 1 & 2
[hider=Why?]I wish Roller-coaster Tycoon 3 ran better. I wish that the games after it weren't garbage. But these old two? They're fucking amazing fun. And getting a roller-coaster with near perfect stats? That's such a good feeling.[/hider]
09 - Sims 2 + Expansions
[hider=Why?]Eh, it's fun. If you know what the Sims is, you know why it's so fun. If you haven't, it's basically do things with people, the game. Like a doll-house, but for nerds.[/hider]
10 - The World Ends with You
[hider=Why?]Great game, fun mechanics, still play it every time I'm out and about with my DS and need to pass time.[/hider]

Honourable Mentions

Black & White 1
Bioshock 1
Civilization 5
Diablo 2
Harvest Moon 64
Hotline Miami
MarioKart 64
Minecraft
Red Alert 1
Shadow of the Colossus
Super Hexagon
Super Smash Brothers 64
Super Time Force
Tales of Symphonia
Telltale's Walking Dead Season 1
Warcraft 2 & 3