My longest time playing is well tied between a few games..
Champions of Norrath PS2: 24 Hours
My friend and I held a competition (two others where with us making a group of four) to see who could get the farthest in the game, and who could get the best loot.
P.S Lost the competition.. Fun thou!
RuneScape Browser F2P MMO: 24 Hours
Another competition on who could get the furthest (we all made a new accounts), 30+ (forget the exact number) people participated but only 11 of us actually stayed up, myself included.
P.P.S. Lost again.. Still fun thou
I mean of course I've spent 8 hours plus per sitting for Skyrim. and the same for some other games but those two are were the most fun.