I never separate stems, just use AI samples that are already separeted. Separation will always result in artifacts, so if you're doing it's better make it MIDI and make it like @shinyzen .

For me tubes and tapes are a must, tapes for nasty high end, tubes to bring dull high end to life. Very often IR's, especially mic IR's. AA Lava plays a big role here. PastToFuture also some useful ones. Sometimes sampler/bitcrusher if i want to make it sound like a sample. Playing LR on Fabfilter Pro-Q followed by Ozone Imager can make some sources more interesting, and less AI-like.

Spoiler: not a rule 4 every1

I abuse Nebula and BYOD (the second one instead of Proteus BC of the oversampling; Proteus can bring even more aliasing if pushed hard) on those AI stuff