Rigby's a special case of fridge brilliance. The attributes that make him unlikable are those of a manchild with a major case of arrested development. I mean, he clearly has brother issues for one thing. But also, he seems the most reluctant to grow up, especially considering that Mordecai has had enormous character development, and shows signs of growing up. Rigby finds it threatening that Mordecai may just be moving on, especially when there's a woman involved. If something major changes, Rigby's world is going to come crashing down, and he acts out as a result. Every other character has been developing and changing and growing, Rigby hasn't. Regular Show, for all it's 80's references and weird supernatural stuff is a coming of age story.
Now here's a character I'm surprised no one mentioned. Allen Gregory. The show with no likable character (maybe the "Magical black guy" adding to the unfortunateness of the series), but Allen just... ugh. In a different time, a different place, with better writers who weren't trying to be so different they alienated the audience, the show could have worked. But as for the character himself... normally, I LOVE an arrogant, oblivious doofus as a protagonist. Inspector Gadget, Hong Kong Phooey... but those characters were likable. The flaws were what made the show funny, and the characters weren't nasty, just deluded, and most of the cast was enjoying that ride. Here, we got a power hungry jerkwad that made Daffy Duck look modest and respectable... and at least Daffy gets his beak blown off every so often. He's insufferable, but suffers for it. Allen just... if the statement that spoiled rich kids are twats, the message was lost in the concept. I like Jonah Hill, but I find this abomination of animation a bigger inexcusable act of homophobic hostility than calling that paprazzi guy the F word (even though Paparazzi deserve everything celebrities put them through).