In general or in one sitting? I can't answer the second one although it would probably be Duke Nukem 64...I once spent between 22 to 26 hours of a regular weekend playing it as a kid. As for overall hours clocked, probably something like...well Duke Nukem 3D. I've played it on the PC, N64 and, XBLA; playing it once every so many months in an extremely loose cycle that began back in about...I dunno, 98? 99? Other games that I've played the Hell out of would be Dynasty Warrior 3, Super Mario Bros, Ninja Gaiden, Super Mario World, Star Fox, Star Fox 64...these are all games I've played since childhood and still find time for. Hell, I pop in Ninja Gaiden and, SMB while I wait for downloads and, console updates at this point!