Straight gaming only [no bathroom breaks/food] I think 9 hours on Twilight Princess.

However despite breaks I would say OoT should top it.... first time I played it was at my friends house and we took it in turns playing from just after lunch (2-3 ish) till about midnight [we had about 45 min break for dinner]. then I slept over and when we got up we were back on the game, break for breakfast, and one for lunch (say 2 hours for both) then we came back to my house and brought the game, break for dinner (1h) and playing till midnight again, break for sleep, playing once more in the morning followed by breakfast and more OoT. Finished in time for lunch.
so basically 48hours where all I did was sleep, eat, go to the bathroom and play OoT.

hmm perhaps TP probably counts more though as it was only me playing, that and the day I spent 9 hours straight on it I also spent a further 7.5 hours on it. though those hours were split either side of the 9.

Isn't the avatar proof enough? :)

^yeah it kinda is but i didnt know it would be one of your favorite series(i have never played one but am considering it)

18 hours standing up in an arcade playing Guantlet with a friend.

I think between 16-20 hrs. <-- i call that Tony Hawk summer!!! lmao

^^proof that Tony hawks pro skater 2 is one of the greatest games ever made (we also played THPS1 for that long too)!!
of course, i took turns with my brothers. and that includes meals and bathroom breaks, too. (we left the machine on while we ate) (we didnt have a playstation, it was rented, so we understandably didnt have memory cards. we didnt want to lose anything and we wanted to make it worth our while to unlock stuff.)

12 hours for Twilight Princess.

8 hours on the original Mario Party.

40h Quake 2 lanparty
I'm not proud of this :(

12+ hours, but never on a single game. Last time i did that was years ago when a friend came over, we rented 1-2 games, and we played like 4-5 of them from 8 at night till 8 the next morning.

I've done 6-8 hour sessions on various other games, but not typically. Typically my max is 4 hours.

26 hours of Quake on a LAN.I took a physics final, drove 300 miles, got there at 3 am, and played until my body stopped responding.