Ok, here goes:
free
TAL - noisemaker : bread and butter synth, useful for trap, EDM, and other genres. I would rate it about 80% accessible. The parameters are all labeled but the value labels are not there.
dexed: a very good free synth. It is an emulation of the yamaha dx7, and can is compatible with sysex files from it, so that means you can find a lot of presets.
Iota mini: a very nice little rompler. Extremely useful for trap and hip-hop, it comes with 50 presets and 10 trap drum kits.
Surge: nice wavetable synth. A lot of presets, but it is more advanced, and not entirely accessible.
Helm: nice free synth with lots of presets, a few waveforms, unison support, nice good effects.
paid
electra2: my one go-to synth, I didn't actually buy it yet but I plan on doing so very soon if I can. It's an extremely powerful synth with a ton of presets and all the parameters are there and labeled. You can also import samples and custom wave tables using OCR in the plugin interface.
TAL - mod: a nice synth. It's kind of noisemaker's bigger brother so to speak. It's at a reasonable price and comes with a fair amount of presets.
I have also heard good things about purity, nexus, and native instruments stuff although I havn't been able to try them myself.
And if you need any more samples and drums, you can try cymatics, w. a. production,
$1,
$1, and trisamples.com
Hope this helps!
Just another thing, if you can't insert midi items, you need to make a time selection with left and right bracket at the start and end in order to set a proper length for the item. This is why I was held up from truly seeing the full potential and power of reaper for over a year and a half, and I hope noone ever makes the same mistake again
And also, u/midi_man has made an absolutely fantastic pack with a ton of midi files which you can get inspiration from.
Let me know if you have any more questions and I will be happy to help :)