Beckett. I hated, hated, hated Keller. They just threw us at her waaaay too much early on and it left a bad impression (though, I got accustomed to her by season 5). Also, her character seemed too much of a Mary Sue (even for Stargate) and I couldn't buy into her character background or the fact that she would be on the Atlantis mission in the first place.
As for the "Keller = Slut" thing...I don't necessarily buy into that, but I think the writers unintentionally did a disservice to the character by having her get into romantic entanglements with another main character in almost back-to-back episodes. Where as other characters that might have had a "relationship/hook up" episode, they were reasonably spaced apart (and were fairly rare even then).
I actually wish they had did more with the love triangle. I think it would have been intresting, seeing McKay and Ronon sort of competing against each other. I don't think this would have hurt the Keller, character unless they did something like they do with Kate on "Lost" and have her pingpong back and forth between Jack and Sawyer, depending on what mood she is in.
Jason