If budget isn't an issue, then take a good look at Quantum Leap Pianos. You WILL need an extremely powerful computer, however. Their recommended system requirements are: Core 2 Duo 2.5Ghz or faster processor, 4GB RAM, sound card with ASIO drivers, 64-bit OS and host. Keep in mind those requirements are to run just the pianos comfortably, with few (if any) sounds from other sources. I'd STRONGLY encourage putting these sounds on their own hard drive for best performance. As of right now, I think these are the gold standard of piano samples.
Going down in quality just a bit, you've got Synthology Ivory and Native Instrument's Akoustik Piano. Both of those will run in anywhere from $100-200 cheaper than QL Pianos, and require significantly less power. They still will need a modern computer, but you should be able to run them fine, even if you don't have a quad core and 8 gig of ram. Aim for a fast dual-core, and 4 gig of ram if possible. They're pretty affordable now, and have really great quality.
Next down the ladder you have smaller stand-alone piano packages, like Steinberg's The Grand 2, and the Bosendorfer 290. They have pretty equal quality to Ivory and Akoustik Piano, but don't include multiple pianos. You can find these from $100-200, depending on where you look. For best results, you'd want at least 2 gig of ram and a modern dual core processor. I'd personally recommend 4 gig for this (assuming you're going to be running it with other instruments), but 2 gig should be alright.
Keep going down and you'll get to the pianos included in general sampling packages. Garritan's Personal Orchestra has one, EWQL Silver has one, and several other orchestral libraries include one. The piano will sound quite synthetic if it's by itself (any trained pianist will be able to tell it's fake very easily), but these pianos are still pretty good quality, and fit nicely into a full-bodied mix. These don't require much processing power at all. If you get your hands on one of these, I'd recommend 1 gig of ram, and a modest dual-core processor (even a fast single-core will do well with these) for the piano and minimal other instruments. If you think you'll be using a lot of instruments in addition to these, aim for 2+ gig and a dual-core.
At the bottom of the ladder are the Soundfont/free VST pianos. I hate to generalize, but I've yet to come across any pianos that fall into this categories that are even remotely convincing. If you have an aging computer, or a very limited budget, this is your category.
If you're a typical user (and have a typical computer), you should find yourself falling somewhere in the middle of the ladder. The Grand 2 is very nice and very affordable, and won't require a bleeding-edge pc. the Bosendorfer 290 is also another great choice, although I'd argue that the Steinway piano is more flexible as a whole. Bosendorfers are wonderful pianos, but they do have a very powerful sound that may not be suited for what you're writing.
Hope you find what you're looking for!