A lot of people seem to like
Wusikstation, but to be honest I don't like it that much. It never ran that well on my system - glitches and skips all over the place when playing with the sustain pedal down - and I honestly didn't think the included sounds were all that remarkable. A lot of power in there as far as making your own patches, but not all that useful as a "bread & butter" VSTi, in my experience.
YMMV!
A friend of mine used to use
Luxonix Purity, which basically emulated a ROMPLER workstation synth in software. No way that I know of to get your own sounds in there, but as a pallete of basic sounds it was pretty cool. I thought it had been abandoned, but the website is showing signs of recent development, so it may be worth a look
Personally, while I do most of my work on a PC with Reaper, I own Logic 8 Express and a Macbook, so I've converted most of the EXS instruments that I liked across to SF2 format, and I play them with the free SFZ plugin. A bit of work to set up, but I've got track templates now for all my oft-used ones, so they're ready to go.