I'm also a fan of Piano One, which is currently available for free with SoundMagic's Neo Piano (the "engine"):
http://www.supremepiano.com/index.html?piano1.html
It's based on a Yamaha C7 Grand, sounds good for general use and is light on CPU.
Note that, after opening an instance of Neo Piano, you have to click the area that says "No instrument loaded" and select the Piano1.ins file that is supplied with the download. After that, you can save it as an FX Chain in Reaper so that Neo Piano always loads with Piano1 selected.