An old PS1 game called Front Mission 3, played through at least 5 times. Brother borrowed the disc and lost it in the end :-( . Tried it in some emulators, but it doesnt feel as good and usually it crashes.
Borderlands 1 and 2 qualify I think. I put about 250hrs into BL2 since release, then burned out about a month ago, thought I'd never pick it up again but started a new toon a few days ago and its been a blast.