@PoisonClaw you clearly haven't fought the blue smelter demon if you think the fume knight is the hardest XD
seriously tho, worst boss is probably the bed of chaos, as it isn't hard, just frustrating, cause you gotta run about everywhere then the boss dies in like two hits. like the fuck i came here for a fight and all you did was swing a giant arm and break the floor.
one of the best would be the ruin sentinels for me, it's a challenge to get there and the fight is three of the sentinels at once, which seems quite cheap but it makes you watch the whole area, rather than focusing on one boss which there usually always is. you gotta whittle them down slowly until you're done.
I'd argue that champion gundyr is both the best and worst boss, because he's there kinda to test what you've learnt to the extreme, and at the same time he is literally a walking rape machine within armour who just cheaps the fuck out of everyone and everything.
on an added note, i don't get why everyone always complains about the capra demon, he was one of the easiest bosses of the entire series for me, i read about him being really hard and was dreading it but i finished the fight in about 30 seconds and i was kinda disappointed he was so pathetic.