Mine was Dragon Warrior on the NES. It was so boring, as it seemed like I had to spend about 80% of the game walking around & fighting monsters to get stronger & get money to buy weapons, with the occasional dungeon here and there. Of course, the dungeons were all pretty much the same. The graphics were terrible (by NES standards), there was pretty much no storyline to follow, and the battles (which was a vast majority of the game) were just choosing to fight/use a spell/run which required no skill apart from highlighting an option and pressing A.
Nonetheless, I played through the entire thing despite it being so dull. It was a step above masochism.