Dragonrend when fighting dragons, of course.
Aura Whisper whenever I'm sneaking, so I know exactly what's ahead of me while moving through dungeons.
Become Ethereal, usually just as a fast way to descend extremely long drops, and sometimes to run past traps.
Fire Breath to cause quick damage if I've got multiple enemies in front of me.
Unrelenting Force, because it is at once iconic, useful, and kind of hilarious.
Whirlwind Sprint to dash past traps or as a means of reaching areas I normally wouldn't be able to reach.
And those are the only ones I tend to use on a regular basis, unfortunately. I should probably vary it a little more than I do. Marked For Death in particular is a Shout that I want to get into the habit of using more often; it can essentially turn an enemy's armor into cardboard, all while slowly leeching away their health. Potentially very useful against damage sponge enemies like higher-level dragons and draugr. Slow Time might also be useful for my mage character. She doesn't have bullet time on her ranged attacks like my archery specialists do, so that could be a good way to compensate and improve her aim (something I think her slightly-crispy follower would appreciate very much).