First I wanted to say the original Assassin's Creed, but when I think about it AC3 might actually top it in the tedium department. The first game at least felt kinda fresh and interesting despite being repetitive as all hell. AC3, on the other hand, felt stale, uninspired and bloated as all hell. Didn't help that Connor is so bland, humorless and unlikeable that even Altaïr seemed appealing in comparison.
Diablo 2 is boring and tedious to me as well. I absolutely love the first Diablo and wish I could get into 2. Everything about it feels so dull. Once every few years I reinstall it to see if it clicks, but I have never made it past the first act. I think I might actually have fallen asleep trying to play it once.