Hey! Welcome to REAPER
Honestly...all of them! There are a TON of JS effects, but if you're new to REAPER (or recording) I would stick with the stock "Cockos" plugins in the beginning, but then again it's always fun to explore and experiment with different plugins!
This is totally subjective to the type of material you are mixing. Some people like some distortion, some like LOTS of reverb, some don't, some use pitch correction, I've even used a bit crusher on some vocals.
Normally what gets used on vocals is compression and a slight bit of reverb. Though, some prefer delay over reverb. The stock "reaverbate" is a great reverb plugin. There is also "reaverb" which is convolution-based.
Again, that depends on what you're mixing. Most often, people don't touch the master bus. If something needs to be fixed, you're better off fixing it in the mix itself (although, some things come in handy. Like tape or vinyl emulation, or an obvious effect that will be applied to the entire project.)
If you're mastering your own projects though, you might want to try ReaComp and the JS effect "eventhorizon2" limiter to bring the levels up. Just don't go crazy. Too much compression/limiting will make your mixes fatiguing, and sometimes even annoying to listen to.
Honestly, there is no magic formula to this thing called "audio engineering." Just use your ears, experiment, and tweak until it sounds good to you. Every mix is different and will be treated differently. There are some great resources over in the forums at reaper.fm including the popular thread "Why Do Your Recordings Sound Like Ass?" and of course the user manual and Geoffrey Francis's other great REAPER books.
I hope that helps!