tricky one.
part of me wants to say skyrim, because for its many many faults it's the only game i've bought on more than 1 platform.
part of me wants to say kerbal space program, because it was the game i wanted to exist as a kid but the tech didn't allow it.
part of me wants to say arma, because i can think of no other game i'd tolerate having such awful performance
however, given all 3 of those rely very heavily on modding for me to really enjoy them....
imma have to say borderlands 2, because it's the most hours i've sunk into any single game whilst playing it as intended, unmodified, and without quitting due to major bugs (although granted some are manifesting now i'm replaying in coop)
shame it's not a worst game of all time list, because i've recently attempted to play black ops 2 and that definately makes the cut for both being inexcusably broken and the short time it was working the game just sucked with about 50% of the gameplay being cutscenes and those cutscenes spectacularly failing to explain the story or be entertaining.....