Must get powers are:
Affect Mind/Dominate Mind: It can really be fun and not to mention funny. Just Force Persuade the thugs on Nar Shaddaa by that central pit to jump in. It's hilarious.
Storm: It may take until level 18 to have storm, but storm has saved my sorry ass more than a few times.
Master Heal: Get it when you can, it can help in your conservation of medpacs and if your a light jedi it is relatively cheap on your force points.
Wave: A lot like storm, use it on a group of enemies and unless they reflex save you can wreak chaos.
Insanity/Stasis Field: Both of them are much like Wave but although they dont damage, the stun effect lasts longer. I'd suggest having both, as both can make up for the weaknesses in the other.
Master Speed: I really rarely use speed, but it's not about being able to run at a million miles an hour, you'll gain 2 extra attacks which is extremely handy on bosses.