I've been buying and selling equipment for almost 25 years so I'll stick to the more recent stuff.
Best: Cakewalk Pro Audio 9 -- it lead to Sonar 1 through 6 (and I can still unlock the mp3 encoder

)!
Edirol PCR-800 -- great control surface, works great with ACT. Not the best keyboard action in the world, but you get used to it. I use it almost exclusively now.
Roland Fantom X6 -- installed the orchestral and vintage keys SRX boards. I'll probably have this synth forever.
William Coakley's Perfect Piano volume V -- loaded into the Fantom X6...it is my go to piano and it blows away the stock Fantom pianos
Yamaha S90ES -- great synth engine, great feeling keys, nice Sonar control surface for those times when I'm working a few feet away from the computer.
Spectrasonics Atmosphere, Trilogy & Stylus RMX -- great stuff...what else do I need to say?
Minimoog V -- pre-ordered this when it was announced. It was a big disappointment at first, but Arturia's updates inspired me to dig deeper and I've fallen in love with it.
GMedia MTron -- wonderful!
Voxengo -- whole bunch of things...best $700 I've spent on VST plugins.
Sonicprojects OP-X Pro -- OB-X emulation that sounds fantastic.
Worst: OBTune -- a watered down, pace-ridden version of AutoTune that never worked on my computer.
SampleTank 2 & SonikSynth 2 -- bought ST2 at Guitar Center and crossgraded to SS2. The quality of the sounds is less than stellar (with a few exceptions) and the source sampling was sloppy. I have had these for years and the only sound I have ever used on a recording was an alto sax patch from ST2...it's pretty convincing.
NI B4 II -- foolishly upgraded the moment it came out. The Leslie simulation just plain sucks. I should have saved my $99 (especially since I have a new Korg CX3).
Roland VS-1680 -- bought this just before computers got powerful enough to record with. It was buggy and difficult to operate which detracted from its usefulness.
Pyro -- never worked...not even once.
I'm sure there are plenty of others in the "worst" category that I just can't think of right now...
Bill