I'd say Gungans, but my problem is with a specific Gungan. Based on that I'm willing to let the rest of their race off the hook.
Honestly my least favorite alien races in Star Wars are Rodians (Greedo's people) and Cathar. Rodians because I've never seen one I didn't want to gun down for being annoying, and Cathar because they serve no purpose other than to be a race of gratuitous catgirls. Totally out of place in Star Wars if you ask me.
At least Empire side in SWTOR I get to kill copious amounts of Cathar.