1. Ultima Online: Stygian Abyss
I used to play Ultima Online back around 2000ish. Holy crapola was this game was addictive. I would have failed my school exams had my subscription not ran out. This game is the reason that I never again touched an mmorpg.
2. Baldur's Gate II: Shadows of Amn
A wonderful game that seemed infinitely repayable. But the time commitment to complete just one play through disallowed me to ever experience the game using a different character class and different non-player characters. Maybe the only game that I would actively describe as too big.
3. The Elder Scrolls IV: Oblivion
Ludicrious in size. scope and ambition. The last time I played this (maybe spring 08) I had sunk 124 hours into this game. But my quest list never seemed to get any smaller. I never even started the thieves guild! How can one simply jump back into a game of this magnitude?
4. Super Metroid
I played this game for the first time in spring 09 and i felt that it held up well. A testament to this is that I stopped playing Bioshock and Fable 2 to crack on with Super Metroid. The reason that I will never play it again? Watch the Speed Runs on Youtube and then you shall understand.
5. Secret of Mana
Twice in my childhood, i played this game almost to completion before major distractions in my life pulled me away. Both times i felt that I could not simply 'jump back in' after a few months away so I started again from the beginning. But again distractions pulled me away. Maybe I was never to know how this game ended.
6. Final Fantasy IX
As with secret of mana, on two seperate occasions I played this game almost to completion. Both times when I neared the end I moved house and both times I lost a game disc. Again, maybe I was never meant to see the ending. I would love to play this game again for the story, but I never like the combat system, finding it monotonous and repetitive.
7. Madden NFL 2005
(Note:Not the DS version, but the xbox version. Could find the correct box art!)
The last great Madden game. After this version Madden traversed an interesting paradigm; the game became incrementally better but at the same time became worse. By this I mean that even though the issues with 05 and its predecessors were being fixed, somehow the fun was being sucked out of the game. 05 was the only game were I feared playing against a team with a good defense. I still remember my New Orleans Saints losing 0-3 to the CPU's Baltimore Ravens in overtime of the Superbowl. That was one of the most intense moments for me in my gaming history. I would honestly compare the intensity to the 'who lives, who dies?' moment in Mass Effect. Seriously!
In December 2008 i picked up a second hand copy of the infamous ESPN NFL 2K5, for the bargain price of five bucks. 2K5 blew all my preconceived notions of a sports video game out of the water and I played it solidly for around six months. For this reason, I know that I could never return to Madden 2005 and have the same experience that I had had in the past.