I thought about making this a poll, but there are so many bosses it would just be a clutter.
I personally haven't finished the game yet (I've got Bed of chaos, Four Kings, the DLC and Gwyn left). The hardest boss so far for me has to be Ornstein and Smough, aka Bigge-smalls/Snorlax and Picachu/etc. I didn't bang my head against a wall over them, but it took me around 9 runs before getting them. The easiest was pinwheel. That... thing, went down in a second when I got to him.
Bonus Question: Did you get any bosses on your first try? I downed Asylum Demon, Taurus Demon, Queelana the spider witch-bitch, Gaping Dragon, Sif, Moonlight Butterfly, Pinwheel and Seath the Scaleless on my first.