Must have first paid VSTs

So I am pretty new to writing and mixing original songs. Been using reaper for couple of years on and off and always stayed with Rea/JS plugins plus a couple of free VSTs. When I upgraded my interface to an Arturia I got som plugins bundled with that (Pree 1973, Tapedelay, plate reverb) and loved to play around with that so Im thinking about upgrade some of the reaper standard stuffs.

What are some "must have" plugins or maybe packs of plugins that can be used for a variaity of tracks. My music is mostly build around guitar (mostly Amplitube but also mic'd acoustic guitar), some synths and SSDrums. Genre is some kind of Rock, but like to experiment.

Maybe a compressors, eq, reverb?

EDIT: Wow, what an awesome response. Impossible to answer all of you but THANK YOU ALL for great input. Im going to test multiple of the freebies you gave as exemples, and maybe I never need to pay for at VST.