I've played Cave Story to the point where I actually don't want to play it anymore. I've done a few minimalist runs, plenty of speed runs, and even more 100% runs. I just can't bring myself to pick it up again.
Pokemon Silver ate months and months of my life, all because I enjoyed wandering around the wonderful world of Pokemon and roflstomping the poor NPCs with my party of awesome looking but probably shite in competitive play Pokemon. I didn't even know there WAS competitive Pokemon play at the time, I just went with the ones I thought were nifty.
Oddly enough, the fishing minigame in LoZ:OoT currently owns several days of my life. It was just so relaxing and satisfying to sit and catch fish for hours on end for some reason.
LoZ:MM also owns months of my life, most of the time spent just screwing around with the different masks.
I played Binding of Isaac almost exclusively for a few months, and I've managed to unlock (not find, but unlock) all the items except for The Shears. Mostly because you need to die 100 times for those to unlock, and last time I played my mom kills and deaths were neck and neck at 74, and I don't really wanna screw that up.
Rock Band was my baby for a solid year and a half. I'd get home from school and play until I had to go to bed every single day. My guitar controller actually has grooves where my fingers would rest on the buttons.
There's plenty others I'm likely forgetting, but currently I'm playing E.Y.E
ivine Cybermancy and slowly grinding it under my heel. I'm level 115, have all the research and cybertech upgrades, and now I'm just fucking with the game to see how badly I can break it without it crashing.