I have to recommend both Battery 2 and Kontakt 2, if u have hardware u can sample the hell out of it with Kontakt, otherwise check out Linplug Albino, any other of the native instruments stuff and Smartelectronix a FANTASTIC :D freeware/donationware company, some of the best plugins outthere imo.
Other than that you should definalty try reaktor as Calx mentioned it is the daddy. :shock:
But at the end of the day depends what purpose u want your plugins to serve, bass synth, pads, good allrounder?