Alright I'm sorry I couldn't get all the worthy nominees on here, but this is one hell of a list to narrow down.  I liked Hercules the best personally.  It just had a real mano e mano kinda feel that I don't get when I'm jumping around an enemy 2-500x my size.  I felt like it was even battle from the outset and the ending was one of my favorite moments of the series.  Also it gives you the cestus which are awesome.  For some reason I kept thinking of Disney's Hercules too so that gave it a comedic quality.

Posidon is certainly the most impressive graphically IMO

whichever one is the big monster.

Hercules

Kevin Sorbo sealed the deal for me with that one lol.

General RAAM!

Posidon best tutorial boss ever. The first time I fight Zesu in GOW3 was epic it was like playing a fighting game. I wish Helious was a boss fight though.

1. Ares
2. Cronos
3. Poseidon

I knew "other" would be the main choice lol I tried to pick the most popular

Cronos! loved that one

oh and the online MP is a BLAST!

Why Poseidon is not in the list?

Most epic boss battle already created.