Stun.
Is this really in question? Stun blocks abilities, so it trumps ability block. Have you ever seen a stunned character assist? Yeah, thought not. All TM gained while stunned has to be regained to go again. They won't attack, so with multiple stunners, you are easily fighting five on four, or three, or two, or one...
There should be no debate about it, stun is king.
If you were going to pick a queen, it *has* to be buff immunity. There is nothing like it in the game and the only way to preemptively stop certain problems. What problems? How about zeta Qui Gon, zeta Maul, or ch.aze? How about the auto-taunt on the RG? Ever watch Yoda try to do tenacity up with buff immunity on him? Priceless! Totally wastes his turn. Same thing with any other taunter or Leia. Ah, yeah - preventing stealth, one of buff immunity's strengths.