Mass Effect
both 1 and 2
I kept telling myself "these are great, this is fun". Took me way too long to realize that, no, they're not that much fun the third or fourth time around. ME3 didn't interest me in the slightest after that, because why would I want to do even MORE of the same cover-based shooting, hacking, endless talking and trodding through city hubs?
Don't get me wrong, I liked them at first, but now they bore me to death. Everything you do in them. No, have to do in them. And that's the thing. Playing them is a chore now, not fun playtime.