Bolo's singing... you even get an achievement for listening to his song all the way through.
A lot of the banter aboard the Argo, especially the conversations involving Pan. A highlight in particular? He and Hercules debating over what team of Greek monsters would defeat the other.
Pan: "Scylla and Charybdis! There! Beat that!"
Hercules: "Come now, Pan! Now you're being unreasonable!"
Achilles' constant jokes are a source of much humor in the game. He makes fun of absolutely everything and anything that crosses his path. It's made even funnier by the fact he's voiced by Crispin Freeman.