PC:
1. Icewind Dale series
2. Tibia MMORPG
3. SPLIT: Unreal Tournament 2K4 or Transport Tycoon Deluxe
Mainly based on overall playtime and replayability. I've played Tibia on and off since 1998, so naturally I've accumulated quite the playtime, I'm also currently playing it a bit, but I'm not much of a gamer any more, and definitely not an MMORPG gamer. UT2K4 is what I boot up when I need to go about killing people rampantly. Transport Tycoon Deluxe, my favourite business simulator, played since it came out on and off in periods, mostly on LAN. Icewind Dale is simply the best game overall that I've ever played (for PC).
Console:
1. Final Fantasy VII
2. Gran Turismo 2
3. SPLIT Soul Calibur III or Grandia
Same general idea, but also what I remember the best. I don't really play a lot of console games, it's mainly Final Fantasy series and various racing games like Gran Turismo, Need For Speed and Burnout. I can't remember half of the PlayStation (X) games I have, mainly because it's been so long since I saw them. There might be a few more of those that should be fighting for the third place and second place here. Either way, Final Fantasy VII is by far the best game in the series and the series itself is excellent. Admittedly, I've spent more time on FFX, but that's because of the Blitzball mini-game and also the vast possibilities to develop characters as you please - my Lulu deals maximum damage in melee while her magic capabilities are limited. Gran Turismo 2, not because it's better than the other Gran Turismo games (I guess they can only get better with each iteration anyway), but because it's the one I've spent the most time on. I played this A LOT with friends in my childhood. Racing to fully tune all the cars first. Soul Calibur III because I like fighting games, some button-bashing now and then is good exercise, and it's different in the sense it has weapons. It's also better than Soul Calibur IV because of the "Chronicles" that beats this stupid tower by a far-cry. Grandia is one of the most memorable experiences I have from my early days as a gamer. Although I can't really claim to remember the story, I do remember the game; how it started and how the graphics was.. a lot of short, mini-events that I still remember. It was great and I've been looking to buy it (my disc is gone, either that or I borrowed it before), just to play it through again.
There's lots of memorable honourable mentions that I should be listing that I will forget, but here are some anyway, so that I don't feel bad about not mentioning them:
(in no particular order)
Tropico 2: Pirate Cove, Dead or Alive, Two Swords, Morrowind, WarCraft III, StarCraft (II), Rayman (I mean, seriously), Pokemon (although I hate the concept now, it was a game I played a lot when the hype came - mainly in school with links, cloning pokemons and shit.. fun times), Heavenly Sword, Jedi Academy, Hogs of War, Worms, Mass Effect, Rise of Nations, Red Alert 2, and more.