Mine without a doubt would be Halo Combat Evolved's (yeah, I know I've been talking about halo alot recently. I only realized recently it was for the PC and I just got it on Xmas, and couldn't play it for about a week. And I'm really excited cause its like my favorite game ever and I can finally play it without going over to a friend's house.) anyway, mine would be Halo Combat Evolved's Assault on the Control Room, (AotCR) level 5. You traverse at least like 30 miles and it takes at least 2 hours. (I've never timed it.) At least a discovered a sizeable non-glitch skip for it. But still, it's a long level either way. But it does shave off like 30-60 minutes.)
In WoW, the Blackrock Depths dungeon is killer
Spent basically a whole night in Zul'Guruub in WoW one time. I miss the guys I used to play with, they were the shiz
and a long time in BT too once, a long with a long Ulduar run. Both 6+ hrs I think.
Any level in Ninja Gaiden on the NES. 'nuff said.
Lego Star Wars 1 a level from Ep.3 it took us 4+ hours with a friend on multiplayer