Easily Riven.
Katarina is likely to beat the shit out of you before you even have a chance to try to hook up with her, and if you manage to, you are dead if anything goes south.
Shyvana has anger issues, the last thing you want is to upset her since next to anything can set her off and I would rather not deal with a massive dragon trying to kill me.
Riven basically is a depressed wanderer that just regrets what she has done and believes in true strength, not people that use underhanded methods to get power. You wouldn't get the time of day from her if you were weak, and if you were strong, she would most likely feel she doesn't deserve to be happy. If you managed to hook up with her, at least you aren't likely to die because of it. I feel Riven is more likely to just leave you than kill you.