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! 
VCV rack is some incredible software
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
Shout outs to befaco for their modules, both physical and virtual
Befaco is incredible, one of my favorites. They also just released Oneiroi on VCV as a $35 premium plug-in, it's on my wishlist. I've been wanting to play around with it since it dropped in hardware, and soon I'll be able to. It's got some differences from the hardware, but from what I've seen it seems to be very true to the hardware while using the fact VCV is software and using that to make it more user-friendly. For instance, being able to load .wav files directly into the looper buffer.