First, I'll touch upon the reasons I feel Shampoo is a terrible choice. For one thing, she comes from a village and a culture where men are treated like second-class citizens—and this is Ranma we're talking about—Ranma, whose name literally means "wild horse". There is no way he would ever willingly marry into that kind of matriarchal society and be happy there. He'd run away in a heartbeat, or piss off one of the elders and get himself banished, divorced, or killed. And that's not even considering the fact that his children would be raised as Amazons, in China, in a culture he doesn't understand, respect, or even like. And two, on the most basic level, Ranma and Shampoo are just not compatible. She turns into a *cat* for one thing, which Ranma pathologically fears; and for another, she's manipulative, clingy, cruel, and has no qualms against killing. Ranma, at his most basic, is a protector, not a killer. I just can't see him adapting to a life where a motto like "obstacles are for killing" is thrown around with aplomb. Not to mention her forwardness makes him uncomfortable: Everytime she grabs him, he looks like he wants to run away and hide! What Ranma really needs is someone who complements him, who can meet him as an equal, and help balance his ego: Shampoo is too strong-willed, opinionated, and set in her ways for any of that.
Which brings me to Ukyo. In all honesty, I feel this pairing is much more realistic than Shampoo. At least with Ukyo, there is friendship and a shared history between the two of them, and their temperaments are much better matched. However, the fact remains that Ranma doesn't see her as anything more than a friend... and not from lack of trying on her part. Ukyo has been very direct and forthright about her intentions, so it's only Ranma who's left dragging his feet. And even if his engagement with the Tendos was cancelled, Ranma hasn't shown any real interest in his love life (outside the ego-boost it provides), and in fact, has been supportive and happy for Ukyo whenever he thought she'd found someone else. Not to mention, in the grand scheme of things, like Shampoo, they're really not compatible long-term. Maybe in a few years, once they've grown and matured... but at the moment, both are too focused on their own desires and goals—Ranma on his martial arts, Ukyo on her okonimiyaki business. Not to mention Ukyo's a little too caught up in her own desires and dreams to focus on Ranma's. Even in her visions of the future, they're very Ukyo-oriented (he travels with her, selling okonomiyaki; he raises the children while she works). And if anything, her idealized vision of their future would be shattered the moment Ranma failed to live up to them—and being the insensitive ass that he is, believe me, it wouldn't take very long. Besides, for Ukyo, I get the impression she's more caught up in the idea of love and marriage than in the actual reality. Now that's she's taken in Konatsu though, and is no longer alone, I think her obsession-edge may start to lessen a bit...
Honestly... do I even need to go there? Can we all just agree that Kodachi is undeniably insane, frightens him, and barring magic, he would NEVER ever consider dating her, let alone making her his bride?