Hellfire Peninsula was awesome. I stayed there all the way to 63, almost 64 on my first trip through back at BC launch.
I liked Borean Tundra, but after doing it on 10-12 characters (plus beta on 2-3 characters!) I just absolutely cannot stand it. The Nexus island area triggers my gag reflex to this day. I find it really ironic that it's quest achievement is called "Nothing Boring about Borean". I beg to differ!