The "Big Three" in keyboard manufacturers are Korg, Roland, and Yamaha. All three have produced legendary equipment, and still make quality gear today. These are the three to look at for your purposes. They each have their own well-rounded general purpose keyboards

Korg: the Triton (and more recently the M3)
Roland: the Fantom
Yamaha: the Motif

All three produce rich, realistic sounds, and which is "better" is a matter of personal preference. Your best bet is to go to a music store and check each of these three out. Decide which you like better. But before purchasing, also realise that each of these have older versions that are no longer sold in music stores. Even the older ones sound amazing, and you can find them (likely on ebay) at much better prices.

My personal opinion: Overall, I like the Triton the best. Korg was the first to produce true sample based keyboards (aka ROMplers) so they have experience. The orchestral sounds are the best. However, the acoustic piano sounds leave something to be desired. Yamaha is a better choice if piano is the primary sound you’re looking for. As for the Fantom, I’ve messed around with it a bit, but not enough to say what its stregnths are.

A few links that might help:
http://en.wikipedia.org/wiki/Korg_Triton
http://en.wikipedia.org/wiki/Yamaha_Motif
http://en.wikipedia.org/wiki/Roland_Fantom_X
References :