Top 10 in terms of time investment, fun, and replayability.
1. Everquest
Holds a special place in my heart as it captivated me like no other game before it, or since in all honesty. It's open concepts, group content, raid content, the ability to pretty much do what you pleased when you pleased, perseverance and the ability to adapt, the community aspect, the rewards (loot) system, the bugs and all the broken concepts, all of it. Not a perfect game by any means, but the experience was one of a kind and one that I will NEVER forget.
2. Baldur's Gate 2 (/w expansion)
The grandess, most epic, replayable role playing game ever created. The sheer amount of content, artistic style, abilities, loot, mod community, all tied together with AD&D 2nd Edition rules set - so much fun. David Warner as the voice of Jon Irenicus, brilliant. The character had to be one of the most superbly written adversaries in any game.
3. Diablo 2
One of 2 games to pull me away from Everquest, the other being Baldur's Gate 2, for extended periods of time. The loot whore in me has never been so happy. And the continued updates throughout the years, unprecedented.
4. The Legend of Zelda: Twilight Princess (Wii)
It's all about the dungeons, the sureal sounds, the gameplay mechanics, the dungeons, the addition of a new/added change in style and concepts, the NPC's, the dungeons. Have I mentioned the dungeons?
5. Resident Evil 4 (Wii)
Holy. Fucking. Shit. What an unexpected, brilliantly paced, ride this game truly turned out to be. Cheesy? Sure! Fun? Yup! The upgrades, the memorable bosses and NPC's, the one liners, the extras, and that beautiful ending music with credits. /swoon
7. Tales of Vesperia
I'm kinda surprised to see this game make my personal list, but I have to tell ya, this game provided so much to do, and most of it was just a blast to play, and replay. I thought the story was interesting, the characters and their back story and motivation to be acceptable and fun. What an unexpected, yet, awesome game this turned out to be.
8. World of Warcraft
Vannila (pre-BC) WoW was a pleasure to be apart of, and experience. Truly, for me, Everquest 2.0.
9. The Legend of Zelda: Majora's Mask
I replay this game now and again and some aspects of this game are just timeless. It messes with my emotions, and creeps me out, and the structure of the game is superbly paced, rock solid, yet sometimes daunting for such an innovative, inventive concept.
10. Uncharted 2
Upped the anti in cinematic experience, certainly one for the books. The humor, the story, the locales and how involved certain respective set-pieces were in gameplay. A beautiful, nicely paced (not to long, not to short), breath taking, fun game with sweet sound.