I am relatively new to virtual instruments, but I have enjoyed all of them so far, especially the Modern U, VSL Steinway D, and Garritan CFX. It's fantastic to have the ability to play various piano instruments whenever I want.
Additionally, the VILabs Ravenscroft 275, which I purchased earlier this week, is also good, though I haven't had much chance to explore it yet.
After reading numerous positive feedbacks about Pianoteq on this forum, I decided to purchase the Pianoteq Standard today. Now, I have the additional Bösendorfer 280VC, Steinway D, and Petrof instruments in Pianoteq. I am particularly interested in using Pianoteq with my iPhone for convenience.