I actually liked the music in Castrum Meridianum better. It's got a Mass Effect feel to it almost. Very spacey\techie\epic, though the end of Prae at Ultima is one of the best boss themes. I'd put Garuda next in line in terms of epic boss battle music.
My favorite dungeon is Haukke Manor - both normal and hard modes though hard mode music is just AMAZING. My first time through there I was super scared, but the tank was awesome, the party was cool, and I just flared all of the things. All was well in the world and now I love that place so much.
All the dungeons have their charms, though. I like Stone Vigil a lot. Who doesn't like dragons? And for just enjoyment of the atmosphere, Sunken Temple for sure.