I have a weird relationship with the Assassin's Creed* series. I own nearly all of them at this point, but I've not finished any of them. And I can't decide which title is worth sinking time into completing. I thought Black Flag would be my jam (I loved Sid Meier's Pirates!), but I got hung up in an endless mission loop where I had to chase a guy through a town or something to that effect, and I sucked at it. (I kept losing him, or getting in trouble on the way, or something. It's been several years since I last gave it a try, so my memory's hazy.)
Origins caught my attention most recently, and I liked what I played of it, but I didn't feel particularly compelled to continue, either. I don't know if other people encounter games like that, where it's fun enough while you're in the middle of it, but it doesn't linger with you and as soon as you set it aside to do something else, you feel no pressing need to continue, beyond a sort of "Well, I bought it, I should probably play it some more."
(* - I say Assassin's Creed, but this also applies to the Far Cry and Watch Dog games. I keep buying them, and I keep getting bored with them. It's a weird cycle.)