Fish Fillet Pack: Blockfish and Spitfish are incredibly useful plugins for vocal mixing. They are my go-to compressor and de-esser plugins.
W1 Limiter: I usually use this on my main vocal track. It helps the vocals sound crisp.
OrilRiver: Good reverb plugin with a lot of nice presents.
Doublinger: "Doubles" the audio kind of like a stereo-separator. I use this on harmonies and -1 octave tracks.
BuzMaxi3: Works well on master tracks to prevent peaking.
Azurite: Chorus plugin that works well for harmonies etc.
BootEQ: Good EQ, but a little confusing to use. I recommend using the pre-sets or a visual EQ instead if you are confused by this.
Charsiesis: I use this for growls and screaming. It works much better than utagrowl and Vocaloid's growl parameter. Even subtlety using it can really make a difference! (Couldn't find a link, but DM me and I can give you my copy)
MCompressor: Nice visual compressor that I sometimes use. I mostly use Blockfish, but sometimes the compression can sound too harsh. I don't really notice any sound differences between REAPERs native compressor and MCompressor, but having a visualization of the functions always helps when mixing.
MAutoPitch: My go-to freeware autotune, but it sounds very obvious (for example, Miki on this cover). I rarely use this on UTAUs, but I sometimes need it for Miki since her voicebank has a lot of pitch bugs. I prefer REAPERs native RePitch plugin for making -1/+1 octave harmony tracks.