Congrats Bit - it's a very difficult choice to be sure.
I'm not sure I could narrow it to one myself, but I have imposed a rule that I won't add to the synth stable till I've either mastered, or discarded the stuff I already have, which includes FM8, Reactor and Massive from NI, Alchemy from Camel Audio, and Tassman from AAS. I really thing this should have me covered for anything I need to do, though I do drool over Zebra. Not sure why, but Synthmaster has never really captured my fancy - the demo songs just don't grab me like many of the Zebra demos do. But, any new synth will have to wait till I've spent a lot more quality time with what I have. I've even put off downloading the Zebralette freebie<G>!
I know that an awful lot of folks really love Omnisphere, and every demo I've heard has sounded amazing. I suspect if I ever had a chance to use it I'd change my tune, but I don't really understand why I need it.
Kontakt is, well, Kontakt, and yes, you really want the OTS Rick bass!!!