For a single player game in recent years it'd be Hotline Miami although I have poured more hours into other games in the past such as Dawn of War: Dark Crusade. It's just that whenever I think of a fun single player experience, the first game that comes to mind is Hotline Miami.
For a multiplayer game it'd be a toss up between the various Halo's I've played and Left 4 Dead. Co-op gaming with friends doesn't get any better than those two but an honourable mention is given to Castle Crashers for being one of the few games I've completed over and over again.
These answers are based purely on the idea of 'fun.' I don't have a 'best game ever' but if I did I'd have to consider titles where fun perhaps wasn't the driving force in playing it.
For a multiplayer game it'd be a toss up between the various Halo's I've played and Left 4 Dead. Co-op gaming with friends doesn't get any better than those two but an honourable mention is given to Castle Crashers for being one of the few games I've completed over and over again.
These answers are based purely on the idea of 'fun.' I don't have a 'best game ever' but if I did I'd have to consider titles where fun perhaps wasn't the driving force in playing it.