Try Blizzard, I find that to be best for multiple enemies. For single enemies, I'd go with Ice Blast, although I'm not sure if it does any good at low levels. I can tell you this much though, that Frozen Orb does suck in hell mode. You can try it out for yourself if you don't believe me. One possible reason might be that it has horrible casting delay, and Glacial Spike has none.

For fire tree, Fireball is now number one, with synergies and what not. It has no cast delay, a bit of splash, and can do insane legit damage.

So this baby seal walks into a club.