I disagree with everyone. The best healer for dark side missions is... NO ONE! Daka doesn't provide enough damage in missions to be useful, she just wastes a spot. Talia's heal is much better, but once you hit levels 5 and 6, especially hard, you're in fights where every enemy has a group attack. So once you use Talia's heal, she's dead, and there goes your 3 star.
So, the best way to clear, at least past 6 hard, on dark side missions is to go no healer. I like to lead with my 7 star Sidious. The crit chance and damage is the key to my strategy. I also have lots of friends that use IG-88, which I select as my support leader. Then I have Phasma for his instant team advantage, my First Order Tie Pilot and Dooku for their multi attacks. My last spot, I change around between a few, but right now Assaj Ventress. Sure, her main special is useless in PvE for the most part, but her group attack with instant CD and Rampage make her amazing at group clears. I have a tendancy to lower everyone's health depending on turn meter, then use her to finish off, that way she's constantly doing a full group attack.
The key for dark side PvE is to end every battle so quickly that you don't need a healer. Light side fighting is much different, you can lead with Luminara and take your sweet time. You also have attack healers like Ahsoka which are awesome. But if you're playing through missions relying on Talia or Daka either one, you're probably going to have to replay several times for 3 stars.