Most people would go with "****" or various racial slurs. I'm personally fine with using them as long as they're not used with actual malice. If, for instance, I'm singing along to Bring the Pain, I'll sing the line "niggas get tossed to the side" without a second thought, but I would absolutely never even consider calling someone that.
As for word I actually do find offensive, I'd go with "egghead" or, if we're including hyphenated phrases, "know-it-all," both of which are used pretty much exclusively by anti-intellectual assholes who need a quick way to dismiss people who are smarter than them.