Norse, though I much enjoy Egyptian and Greek. Roman sux.
Definitely greek. It's just so creative. Roman is a copy of Greek, except different names.
Grek, Japanese, and Egyptian, with a little Sumerian thrown in for good measure.
Native American mythology. All of it, but Aztec, Blackfoot, and Cherokee stand out to me more than others.
Because the others are too mainstream Because it's creative too.