Another good instrument collection that includes a few good synths is the one from AIR Music Tech.
The upgrade to the complete bundle, from any one AIR instrument, is now available for $60 at Audio Deluxe.
https://www.audiodeluxe.com/products/air-instrument-expansion-pack-3-complete-upgrade I got this last summer, and it quenched my desire to spring for the NI Komplete deal, LOL!
It includes Loom, which someone mentioned earlier in this thread, and that is a very cool synth. It is additive synthesis based, so it is different from most. VacuumPro (a subtractive VA type synth), and Hybrid 3 (a hybrid wavetable and VA synth) are also included. Plus you get piano, B3 organ, Rhodes/Wurli digital piano, a sampler with a 37GB library, drum machine, and a few other things. Great bang for the buck! The included presets number in the thousands, and many of them are useful, designed by professional sound designers.
If nothing else, it is a cheap way to fill up your hard drive, and keep you busy for many months!
The cheapest way into this upgrade deal is to look for AIR Xpand!2 on sale. It has been on sale for as low as $1 recently, but don't see that deal right now. Also many MIDI controllers by AKAI, Alesis, and M-Audio come bundled with a couple of these AIR synths. So if you are thinking about another controller, that is another way to add some sounds, and then get the whole bundle cheap.