In regards to personality, Model A. Out of all the biometals in both games, Model A's personality feels the most human, and undergoes the most character development.
As for playstyle, I'd have to go with ZX. I can absolutely shred all the bosses with it regardless of if I'm playing ZX or ZXA. Model Ox is fun too with it's infinite overdrive, but it's kinda cheaty in that regard and I'm not a big fan of how you can't run and attack at the same time while it's active.
If they ever make ZX3, I hope they make it so you can use Model Z by itself and make new models representing Protoman, Bass, Vile (X series), Sigma, Copy X, and Craft. It would also be nice if we got to see Prometheus and Pandora's modified Model W fragments, assuming they're even still alive.