Street Fighter 4.
I've played that game across two different platforms and many, many discs and multiple iterations for probably 1,500 hours.
It definitely affected my university grades, unfortunately, but thankfully I had friends in real life who played, too. I also made quite a few GB friends, so it wasn't a totally pointless experience. Regardless, I can't stomach even a few minutes of it at this point. Installed Ultra recently and played a single match, then promptly uninstalled.
I will never be into anything as much as I was into SF4, and that's OK.