I'm sorta torn between Rancor and Uxibeast, for different reasons.
Rancor because, well, other than the fact that they're frackin' sweet, I still remember the first one I spotted roaming free on Belsavis many, many moons ago (on my first character). I stopped running around and just stared at the hulking beast for a minute before actually saying (out-loud), "Yeah, I'll have me a piece of that..." *
Was the first time I remember using my CE Holocamera to take a shot of the kill, before harvesting it's bioanalysis gooey center.
Uxibeast simply because... well, I don't know why, really, lol. Every time I see one of those spiky cows, I exclaim, "UXI!", and have to run my speeder through them. Same goes for the Icetrompers on Hoth (well, the speeder thing; I don't cry out "uxi" when I run through the trompers; that would be just plain weird!
So, probably Rancor, because there's nothing particularly special about the Uxibeast, but that won't stop me from charging through them.
* This was quickly followed by (again, out-loud), "For LUUUUKE!!" as I attacked the beast