Legend of Zelda: Ocarina of time - I've got to the point where you become an adult and a bit further before I gave up on the game, i just could not find any fun in that game.
Knights of the old republic, Neverwinter nights, Baldur's gate, Dragon age, planescape: torment, Final Fantasy - none of theese i could pull through long enough to find fun, i would have blamed the fact that i don't like turn based games, but i picked up my old Chrono Trigger disk a week ago and found it really fun, so i have no idea what the problem with those games is for me, but i could bare through any of those games to reach the end.