Most time in front for a single time was prolly Quake 1, back many, many years ago... I'd say over 20 hours of non-stop playing.
After that, t'was Ultima Online no doubt, that one I went 10 hours straight eeeeeeeaaasy, many many times over...
So, first person shooter and MMO.
Oh, and CS had me sometimes doing all-nighters (8 hours play).