I have a few. The first one is Maplestory. I played this game a lot when it came out and was glued to my computer everyday. Especially when 2x exp cards were released I spent a crazy amount of time & money trying to level my character. Leveling used to be so slow in that game, especially since I tended to play solo with quests instead of dungeon runs. Then the Big Bang patch came out and other changes slowly pushed me away from the game. Once in a great while I'll boot up the game to see what's new and to see if I could get into it again but so far I can't seem to be able to.
The 2nd game for me was The Elder Scrolls Oblivion Game of the Year Edition. When I first bought my playstation 3 I picked two games I knew nothing about so I had something to play. One was that game and the other was Dragon Age Origins. Up until that point I had never really played a game like Elder Scrolls before. At first I was overwhemed but then once I realized how open the game felt I was playing the game all the time. I played it every day until Skyrim came out which was a couple years later.
And finally the 3rd game I want to talk about is Perfect World. I joined this game a couple weeks after it opened and really liked it. I tended to play quests solo like I normally do when I took a chance and played with some people who over time became some good friends that I played the game with everyday. They invited me into their guild and helped me level my character and would craft gear for me. They even taught me about the different group roles for dungeons and that was when I learned my character was a tank and how to play as a tank, a word I have never even heard of at that point. I also spent a crazy amount of money in their cash shop because of the fashion, exp, and teleport stones. I hated that when fashion is purchased in the shop you get a random color instead of being allowed to pick what color you wanted.