No synth, hard or soft, can do a very convincing piano. Samples or sample-based VSTs are the way you probably wanna go.
I don’t really mess with s/w too much, but from what I’ve tried, Edirol Super Quartet had the best pianos. It’s not just pianos though, it’s got electric pianos, basses, guitars and drums too.
As far as samples, I was pretty impressed with the Miroslav Vitos libraries. William Clokley is supposed to be the shit, I never tried it though.