I think River is probably heterosexual, but I don't think that's a hard line. If she met 'the one' something as unremarkable as their gender wouldn't matter to her.

River isn't likely to think about anything remotely hit list like but I can see the following (were they even remotely close to her age).

* Porthos du Vallon, because he could probably hold his own if they fought and that means he's not easily breakable.
* Erik Lehnsherr, because he would understand about what was done to her in ways other people would not.

As for anyone in the universe, I will always Jayne Cobb because I am a bad bad woman who should probably be in the special hell.