I like nearly all of Apple’s $99 Jam Pack add-on sound libraries for GarageBand (Voices doesn’t do a lot for me), but I also like free. And when I think free and GarageBand I think SoundFonts, E-mu ...
I have a SB Live! and while I can load huge soundfonts (100+ MB) into memory, it is clear while listening to some midis that only a some of the instruments loaded. I believe the SB Live! had a limit ...