I've tried quite a few, and here are my personal picks:
Absolute Favorites
Ivory 3 German D - Best overall combination of playability, features and tone, while not necessarily being the absolute best in any single area. Almost as good as Pianoteq in terms of playability but with a much more pleasing and realistic sound, although with a very slight hint of artificiality. Best una corda samples in any piano VST I've tried. Very well designed GUI.
VSL Synchron CFX: - Drop dead gorgeous tone, fantastic playability, great features and GUI. My personal favorite of all the VSL Synchron pianos. Only misses are no real una corda samples, sub-par sympathetic resonance. Everything else nails it though. I could happily live with either this or Ivory 3 even if I didn't have any others.
Runners Up (Also Fabulous)
VSL Synchron Steinway D-274 - Pristine sounding Steinway Concert D with great tonal charactaristics and excellent playability, though a bit fussy at times. Tons of color and tonal variation between ppp and fff, though some find it a bit bright and hard at higher velocities.
VSL Synchron Bosendorfer Imperial - Beautiful capture of the Bosendorfer tone, fantastic clarity and richness, great playability.
VSL Vienna Imperial - Another Bosendorfer Imperial, but with a slightly warmer and more lived-in tone than the Synchron, possibly even better playability, and real una corda samples. Only let down by some slightly sour tuning, and an odd "twangy" quality in the treble region.
Garritan CFX - Along with Ivory 3 and Modern U, best playability in any sampled piano I've tried. No real major flaws, although I find the tone a little too plain vanilla and prefer the VSL CFX in this regard. Otherwise, this is great.
NI Noire - Totally unique and gorgeous tone, fabulous character, really good playability - my favorite piano on Kontakt. Just wish it had an una corda pedal.
Honorable Mentions
Modern U - Miraculous playability and sound, and easily the best sampled upright on the market. Problem is, I almost never want to play on an upright.
Production Voices 300 Grand - In stock form, this is an also-ran with some serious balance issues and noise buildup. But with some custom user-written bug fixes and a denoised sample set, this becomes a really compelling instrument.
VSL Fazioli F308 - Very unique tone with amazing clarity and warmth combined with little to no color or timbral variation. Ideal for certain types of music (baroque, contemporary), and less so for others (romantic, impressionistic). But if you want a certain type of clean, flat, mellow sound, this does it better than anything else.
VSL Bluthner 1895 - Fantastic character instrument with a uniquely silvery, delicate tone ideal for 19th century period repertoire.
Galaxy Vintage D - Beautiful vintage Steinway with a very warm, comforting tone. A classic that still holds up well despite being quite a few years older than most others on this list.
Melda Productions Meldway Grand - Another beautiful Steinway with a somewhat vintage vibe, let down by some very quirky and annoying software. If this was on another platform I'd probably use it a lot more.
