Canon tells us almost nothing about Kasumi. That doesn't strike me as the safe choice... Devil you know, and all that Jazz. Same goes for the twins, only more so.
Ukyo is the 'safe' choice, as I see it. Aside from her rather disturbing fixation on Ranma, she's pretty mellow and friendly. If you don't mind being in the okoomiyaki biz, or having a partner that crossdresses, it could work out pretty easily. I could see myself being okay with that.
Akane actually strikes me as the most mentally stable of the lot, based on canon evidence. She's the one we actually see doing "normal highschool girl" things. No martial arts or cooking fixation that dominates her life.
She also doesn't come with a load of strings attached like Ukyo and Shampoo, and as such, might actually be willing to pack up and move to Hawaii (she's not personally obligated to inherit the dojo, AFAICT). No need to worry about being trapped in a backwater Chinese Village or shackled to a Okonomiyaki business.
Kodachi just plain creeps me out and I really don't see Yamato Nadeshiko Kasumi being the sort to welcome cultural readjustment. So the "move to Hawaii Plan is limited to Akane or Nabiki.
Okay, yeah, Akane does have a lot of insecurities, but they all strike me as things that could be dealt with, if her buttons weren't constantly being pressed by a jerk with no tact. Akane's temper is vastly exaggerated in fanon anyway.
Nabiki... honestly, relationships require trust. That alone pretty much rules out Nabiki, and Kodachi, and possibly Shampoo, though I'd have to recheck canon to assess how trustworthy the faux chinese girl is.
So, Akane or Ukyo. Probably Akane, as I'm not terribly enthused about the food biz.