"Soft synth" is too generic to truly recommend so I'll take it as "Virtual Instruments". I would highly recommend those:
GForce M-Tron
A fantastic, beautiful sounding and absolutely autentic digital soft version of the Mellotron. The sounds are NOT emulated but transcribed from original 'tron tapes. A must.
GForce Minimonsta
A wonderful emulation of the Minimoog. Very powerful sounding and with expanded sound control capabilities.
Native Instruments B4
Excellent Hammond organ emulation, including Leslie speaker, distortion, etc.
Native Instruments Akoustic Piano
Very cool classical pianos recorded acoustically.
Propellerhead Reason
This is a many-in-one proposition, it includes very fat sounding synthesiser (great sound quality), a very cool visual interface to build your rack, plus you get a sampler with all kinds of sounds including pianos (acoustic and electric), a full orchestra, etc. Fun for weeks...
Good luck!