I hate to say this but probably Morrowind. When I first played it I was confused, frustrated at the fact that I didn't know what to do. But as I played, the game grew on me. Sure the gameplay is pretty dated and the combat sucks but the game has a charm to it that I just fell in love with.

Currently on my second playthrough.

Kingdom Hearts Dream Drop Distance. I love Kingdom Hearts! I got this game as soon as it came out and played it for about a hour and that was it. I hated the drop between characters and it felt like a big constant fight. Last week I came across the game sitting on a dresser and tonight I figured why not try it again. Hours later I am sucked in. (Not saying I will not lose interest again but im hooked right now.)

I've met a lot of people who actually think AoE 1 is better than 2.
I had fun with both, though.
Also, starting with BG2 can be quite confusing since there's no real tutorial. It starts in media res and it was clear that the game was designed for players experienced with the first game.
Fun fact: the romances were introduced because Bioware realized the fas were modding them into BG1 (or so I was told)

I agree with Oblivion. It's easy to find it quite so-so after Morrowind, but I've learned to have fun with it :P

Dead Space.

The demo really turned me off the game, it's a horrible representation of it. I was then given the full game as a gift one Christmas and was blown away by it compared to my first impressions from playing that demo.

Only one so far, Diablo 2 but that's because in my first playthrough, I was doing it all wrong.

Neverwinter Nights might be another one of those games. After finishing and being utterly unimpressed with the 3 offical campaigns, I will be looking at user made content from what I here is leaps and bounds ahead of the OCs, I just haven't tried any of them yet.

XIII

Neverwinter Nights.

I bought it when it came out and loathed it. Years later I got the supermega edition here on GOG bundled with expansions, and decided to play it LAN with my boyfriend and I found myself really enjoying it that way. Slowly but surely playing through it bits at a time.