I'll have to go with Luka. She has the facial experessions that go with her actions that are just absolutely too adorable to ignore. Like in the third version of the Gokaiger ending, on Gaoranger, she does the whole claws and rawr face that's so fucking adorable. And in the episode with the Sneak Brothers (lol), you can see the glint in her eyes when she licks her lips and goes after the manju buns or whatever they are.
Ahim, IMO, is too prim and proper monotone that it's kind of boring to stand. Granted, yeah, she's a princess, I get that. But in every episode I watch of Gokaiger, I can't seem to stand Ahim. I had a little respect for her in episode 13 when she got "kidnapped" and lied about it, "sweetly" placing the blame on the kaijin of the day, but overall, I never find anything she does unique in any episode compared to Luka.