Best distortion or saturation plug in ?

Soundtoys Decapitator! I gravitate towards this - it can do well in most scenarios.

Slate VTC is also really nice but I’ve been off that sub for years now.

I like fabfilter Saturn, esp on vocals. Blackbox is cool on the mix bus.

Kush Omega Preamp N, A, or 458A if I need to make a tonal shift in the sound. The Kush Omega TWK is a little more linear in tone but does great to round the pokey stuff.

The drive control on my Softube Console 1 channel strips are pretty good. You can really push hard into the Summit Grand Channel drive and get some usable saturation. Feels gentle, helpful, and good until you decide you want it to be something else.

The Softube Saturation Knob with the gain matching feature is good, too.

I don't use it often (I don't think I've used the plugin enough to feel good about the amount of time it takes to get results...and I'm in a bit of a rush on some projects right now) but FabFilter's Saturn is good and in everyone's recommendations.

I'll second all of these (except I don't have Tupe), and add PSP Saturator.  I think it gives me the smoothest saturation I've heard from a plugin.  

If you want a plug that does gross distortion with aliasing to destroy electronic stuff, I like Front DAW.

I kinda like AIR distortion

Ah yes, I forgot the Fuse plugs. All of their VPRE ones are really nice! Just got a few of these, recently. The VPRE-2C in particular was getting a workout from me on recent mixes.

Fuse / Neold stuff are my go-to.
Have been digging Arousor lately as well. I love that Dave Derr shred.

Does anyone use the UAD Culture Vulture? I’ve been going back through a lot of my plugin collection since I got better monitors, and I remember the CV being something I previously demo’d and gave up on. Also been meaning to spend some quality time with the Black Box HG2MS for the same reason.

Tone Projects Kelvin

I use the Tchad Blake Decapitator bass preset a lot...

I'm really digging the saturation in the KIT BB N1105. Haven't spent any time with their free Burier saturation plug yet, but I've got it ready to try out. I've also got to try that free GSatPlus by TB Pro Audio - I like their DSEQ a lot.

I also really dig:

• Tone Projects Kelvin. Great plugin.
• Kush Omega 458A. Yes, it changes the tone, but in a way that sometimes eliminates the need for EQ for me.
• FF Saturn 2.
• Acustica Cream. Wow, I love some of the preamps included in this. They sound right to me.
• Cupwise AX102 Tape for Nebula. It can be very subtle. Like tape.
• Softube Tape. It's been awhile since I pulled it up, but it adds a nice "polish" to things. 
• Lindell 80 Bus. It's good - better than Slate VCC.
• Sonnox Oxford Inflator. Still sounds great when it's right.
• Acustica Magenta. Sounds rich and refined, like the Manley gear it sampled.

I haven't tried Decapitator in a long time. For some reason, it never made the final cut for me when I would try it on a track.

I thought Burier was surprisingly horrible, but YMMV!