Even though I do enjoy the cutscenes and I am sure that DMC has an excellent story etc, I am more of a gameplay kind of guy.
From DMC4 in terms of gameplay and mechanics I would say the best designed boss would have been sanctus or dagon. Not because of difficulty, but because being better at the game rewarded you more.
Credo was retardedly easy, you could do anything and easily beat him, dante was a retarded fight I only managed to beat him like 3 days ago with no damage, I think that even though he's cool, the fight was nearly impossible to do in any real stylish skillful manner. Killing the swords on agnus was too easy an exploit, and just an all round really easy boss fight. Bael you can kill in about 40s without even spamming a move. Agnus on mission 6 was boring and annoying.
Of course I still dislike a few things about the other bosses, dagon seems to becomed weakened almost randomly, and if he does the move where he spits out ice (but the completely instant one that he does immediately after you hit him) it's almost unavoidable unless you have DT with nero. Sanctus sometimes hits me cos as he CONSTANTLY FLIES AWAY the camera angle changes suddenly and i run into one of his little orbiting shield ball things.
In terms of what I found coolest, bael was really cool, and if for some reason the saviour got resurrected that would be sweet (if they upped the speed and power of it maybe).