Ok my two cents:
Halo, definately is one on the most overated series out there. My reasons? i found the single player very boring. Didnt like how it felt when you shoot. I didnt like the way microsoft allowed bungie to release a sub HD game on a HD console, when they insisted on every other company make their games at least 720p. Halo 3 was not even close. ( Although i do know microsoft have allowed loads of titles since then sub HD, but at the time they didnt).
I always found the gameplay shallow in single player, but i did really enjoy multiplayer.
COD. I was a huge fan up until black ops, and then i realized how stupid i was to keep giving activision 50 bucks for the same game, year in year out, its the same reason i stopped buying fifa and pro evo and guitar hero, it got ridiculous.
Elder Scrolls???? now i have to admit that im not an RPG fan, i find them boring, ZELDA is probably as far into RPG as i would go, and i konw its more of an adventure game than RPG. But i found Elder Scrolls very dull and boring, even graphically.
Gran Turismo. I just found the driving mechanic boring. Slow paced.
Zelda OOC (just ocarina, not the series as a whole). I am a massive zelda fan, one of the biggest, but i simply cannot warm to this game. I konw most people will think i am mad, but i really didnt like it. I found the dungeons too small. Camera was a huge hinderence. Some very very obscure puzzles that didnt make sense and i found myself completing them mainly by accident. yet i love the rest of the series,
Again, only my own opinions, not trying to offend anyone who loves any of the above, everyone has different tastes. I for one love Mortal Kombat, but im sure there are plenty of people who would smack me for that.