TFP special needs an Omega asap. Vader's Force Crush (Speed Down is huge) second. Not sure what next. All of Palp's abilities need an Omega, but Nihilus' basic and Drain Force seem important to Omega too. Shoretrooper's unique is another good Omega, but probably not as urgent as the other ones. Best order is probably something like:
TFP TIE Strike
Vader Force Crush
Palp Power of the Dark Side
Nihilus Basic
Nihilus Drain Force
Vader basic
Palp basic
Palp Let the Hate Flow
Shore unique
TFP basic
TFP uniqe
Nihilus Annihilate
Vader Culling Blade (not important at all)
Shore taunt
Shore basic
But I haven't played a lot with Nihilus, and probably won't until he's farmable unless I move to a Sith team under a Maul lead. Either way, if you find you need to annihilate Boba a lot, then omega on Annihilate becomes more important. Good luck!