Actually, I disagree with your reasoning. I think many, if not most people think that the main reason why Buffy and Riley's relationship fell through was because she needed someone who was nearly her equal in regards to strength, but I think it has more to do than just the "superpowers or not" aspect. While having a partner with super strength does benefit Buffy in that she doesn't have to hold back all the time and she can have super-sex, I think the main reason for the lack of spark between the two of them was that Riley didn't seem to understand the whole basis of the Slayer thing. He couldn't relate to what it was like to be so much more than human, yet human at the same time, but more importantly, he didn't understand exactly how much her Slayer side affected and defined her. That's why she always tried to keep it a secret from him when she was "hunting" instead of just patrolling, like in Buffy vs. Dracula and Out of My Mind.
Which brings me to Angel and Spike. Unlike Riley, they knew how much being the Slayer shaped Buffy...why else would Spike have always referred to her as "Slayer," often in what could be considered an endearing way? Still, they were more similar to Riley in that they didn't understand what it was like to be both human and more at the same time either...while Riley can't relate to her demon side, they can't relate to her human side. That's why, in the end, Buffy's relationships with Angel and Spike both failed.
I guess in a way, the ideal partner for Buffy would be another Slayer, but unfortunately for her (and the other Slayers (sw)), she has a strong preference for men. So I guess in a way, that would make the ideal partner for her Connor. Now, I know that most people would hear the notion of Buffy/Connor and go, "Ew, gross! She frakked his father, for god's sakes!", but think about it...they're both humans with a demonic side, they both have superpowers (although Connor does appear to be quite a bit weaker than Angel, Spike, and Buffy, the difference definitely isn't so dramatic that she would have to be careful with him all the time like she was with Riley), they both age, they both have a tendency to be very angsty...really, he's the only guy in the world that Buffy could have a long-lasting relationship with.
