The one that turns them to ice, lowish cooldown and really useful

Storm call. It's useful in taking out large groups of enemies.

Become ethereal. I can use it to get past spells and I can use to jump off mountains.

I also like marked for death. It seems to weaken enemies some.

Whirlwind sprint is useful for getting past obstacles and running or for a quick attack.

Personally I like Ethereal because hitting it by accident has yet to kill me or yield a bounty.
But yeah, Freeze! and windsprint are pretty useful too, for a muscle-bound cretin like myself.

Dragonrend seems to result in the dragon flying around until it can find a landing spot on the other side of the mountain from me :toughninja: