Building a collection of VSTs, what are the staples?

If you want my suggestions... Slate VMR is a must own. It's just brilliant. It's ending up replacing everything else I used to mix with.

I'm a big UAD fan so my favorite plugs are all UAD, but if I don't want to use DSP, Slate has become my first choice.

UAD's tape sims are IMHO the best on the market, then next step is Slate VCC/VTM.
 
I haven't seen Fabfilters Saturn mentioned yet. That has become an absolute essential saturation plugin for me, I try and use it with Soundtoys Decapitator, I find they compliment each other very well.
 
As far as free plugins, I gotta say BootEQ and DensityMkII are essential. Density is absolutely incredible for drums, it makes parallel compression massive. I haven't found anything better for it.