Mine is Native Instruments. They have so many great affordable products that help me make my music. I just bought Evolve Mutations and it kicks bum... I love Kontakt 3.5, FM8 etc. NI are always updating their software (thanks for MIDI control in KORE!) and always seem to have something new. The brilliant marketing of the free Kore and Kontakt players are great. If you have a few bucks to spare there is always a great KORE library for well under $100.
Anybody else feel like chiming in?