I mean obviously there's Serum, Vital, and Phase Plant and all the popular ones, but what do you find yourself using?
My current rotation is:
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Phonec by Psychic Modulation. Really dreamy synth meant to sound like a nature documentary from 1984. Very Boards of Canada.
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Repro-5 by u-he. Really nifty emulation of the Prophet 5.
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VCV Rack. The best software version of a modular synthesizer rack around. You can get lost for hours in sound design. Also it's free!
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Dawesome Abyss. Honestly a little finicky and difficult to use but can make some super thick pads.
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QasarBeach by Adam Strange - It's free! Super crunchy recreation of a Fairlight CMI IIx. Probably the most impressive free softsynth I've ever used, and it's more than that! It has a tracker, waveform editor, sampler, a ton of stuff. The GUI is just beautiful. It's really really hard to use and clunky, but it's just got so much soul and love put into it that I can't help but poke around in it sometimes.
What do y'all enjoy using? Also this is a great comm idea and I hope we have lots of cool threads! 
it really is just beautiful at how deep and involved it is. The community is a little...eh, sometimes but there are some very passionate and dedicated creators who I adore. Just pumping out modules for free for us
yeah, i vaguely recall some community ehness recently. I hope they've done better