99% of what I play is piano-centric with no vocals so a great piano sound is of primary importance to me. I look for a nice clear overall tone with a full midrange especially in the soloing area where many piano emulations tend to suck. Also I look for smooth sample transitions across the keyboard.

By far the best piano sound in my collection today is the Steinway B in Colossus. That's probably why I am so keen on trying to integrate softsynths into my live performances. But there's a balance between perfection and practicality and I'm not yet ready to go the lengths it would take to use Colossus on stage... "yet" being the operative word - I haven't ruled it out.

I still rely on the Yamaha PLG150-PF piano expansion card in my 9000 Pro for my live piano sound currently. Not only do I like it but I get a lot of positive comments from other pianists about it. There's very few digital piano sounds I like better, but I am determined to go back to playing an 88 note weighted or grade hammer action keyboard in the near future so the 9000 Pro/PLG150-PF's future with me is limited (although I have considered using the PLG150-PF in an S90 or S90 ES).

The best piano sound and keyboard feel I've experienced lately in a hardware keyboard came from the Casio PX-555. I was highly impressed, especially since it's a fairly lightweight $899 arranger, but the 32 note polyphony killed it as an option for me. I look forward to seeing Casio improve upon this further in the future.