Ah yes, Least Favourite Boss Fights... I can name a few on the top of my head:
Ultros (Final Fantasy VI) - I really like him as a character because he's a reoccurring boss, but it's just the fact that he's literally just the same except has higher stats whenever you face him. Least favourite just because of that. Otherwise, he'd probably be my favourite.
Queen Myra (Gears of War 3) - Normally, I wouldn't dislike this fight purely because it's so great how it's created, mechanics wise. But I hated it because me and a friend tried to do this on Insane to finish the entire Campaign on Insane, and We. Absolutely. Struggled. The only reason we managed to beat it was because for some reason the game had glitched out where no enemy reinforcements spawned, it was just Myra. Thank god for that, otherwise we'd of never of done it.
Black Heart (Hyperdimension Neptunia Re;birth 1) - This was when I was new to JRPG's and encountered her fight in the story for the first time and lost so many times. She's so strong compared to you at first if you don't grind some levels, especially when one member of your time isn't very tanky. (*Cough* Compa *Cough*)
That's all I could really think of at first glance really.