Helm and Vital are both free and really good. I'm also a big fan of dexed, it's a DX7 emulator and you can find a ZIP with 3000 presets.
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Cardinal is my favorite. It's a fork of VCVrack and can be used as a plugin in Reaper.
Sometimes I compose entire pieces in Cardinal and then route each sound to its own track in Reaper. Other times I just use it as a drone or chaos noise machine.
Vital, Surge XT, Viking VK-1, Odin 2 - all are quite solid.
Have not tried Viking. Thanks for the suggestion! (Vital, Surge XT, and Odin2 are solid)
Can’t go wrong with the venerable Synth1!
if you are into more vintage sounds I recommend checking out TAL U-No 60. its meant to resemble the Roland Juno 60. you can get some really juicy old school sounds out of it and it's pretty easy to use.
this next plugin only helps work flow if you also play stringed instruments. the default plugin, Reatune, works as a tuner. it works pretty effectively and I keep it in my amp sim templates.
Reatune is so good. I use it on all my vocal tracks as well. When I'm singing it's nice feedback to see how off I am and then also a good way to correct those mistakes if I'm feeling lazy.
If you could control the pitch it corrected to with MIDI that would be even better. The manual correction is fine but takes longer.
I can't believe I never thought to use it for checking my vocals. I'll have to try that next time.
SurgeXT amazing for a free synth