Personally I don't think any arranger keyboard has a realistic grand piano, the G-70 is my favorite (at least if it has the exact same piano sound as my Fantom XR), But still not good enough.
I've played the Promega series and it has the most realistic piano sound of all hardware keyboards (not softsynths), although I think that the upper range of the Steinway piano on the Promega sounds a bit weak (sounds too thin), I almost bought one, but I went for a PA1X instead(Not a digital piano, I know, but it sounded soo good ), maybe I'll end up buying one anyway, still want a stage piano .
If you really want a superb piano sound, you should look at gigasampler and kontakt instruments, they generally sound much better than any hardware keyboard, my personal favorite is the Virtual Grand Piano by Art Vista .

Greetz ,
Marcel