The stereotype that all men act like cavemen. I'm an intellectual, intelligent, I love thought and creativity, and I will absolutely, under no circumstances, use the whole "I'm a guy, you should have expected this out of me" excuse on myself or anyone else. Bill Cosby did a segment about this, actually. Very funny, and sadly accurate. Many guys think this is an okay way to act, but it just gives women another excuse to hate men for being stupid and ignorant.
I hate the whole "sex for nothing other than sex makes you a slut/douche" stereotype, for both males and females. Two consenting adults, no one else's business.
On the other hand, I hate the "all guys think about is sex" stereotype. Sex is a big part of my life. So is food. So is science. So is worrying about my bills. So is feeding my gecko. Get over it.
Also, the "guys only love thin, blonde women." I love chubby girls (and no, not a just a little pot belly, I love big, curvy women) and brown hair/brown eyes is the sexiest combo ever. It's funny, my fiance is thin, blonde, and green-eyed. But I love him dearly.
So many stereotypes out there that I really do not care for.