+1 on everything from Valhalla and Madrona Labs. Money well spent.
Besides those, my favorite software instrument is the good old Korg Wavestation. It is one of the rare occasions when a plugin is better than the original hardware. And even today it has a very unique sound that is hard to recreate otherwise.
I don’t even know what it costs these days. Everytime I update my computer, it is the first plugin I open up to make sure it still works.