my experience only!
On the input side microphone selection and placement is the most important factor, followed closely by the preamplifier, and not quite so closely by other processing (e.g. compression & EQ). I have an Apollo Twin, and I have to (grudgingly) acknowledge that their Unison(tm) voodoo works well. Preamplifiers that run as Unison plugins do sound different than when run as regular old plugins. No idea why, and while they are cool I still end up using hardware preamplifiers most of the time.
Tracking is a different animal entirely. All of the sudden other factors become important - most notably repeatability and recall. If I'm using a compressor while mixing it is almost always a plugin. Even if you ignore the extra work required to get signals out and back the difference is small and - especially in the context of a mix - I can't hear enough of a difference to warrant the extra effort.
Execpt for reverb... and I can't explain this either, but I have a PCM-90 in the rack, which is certainly not the flagship, and I prefer it for the overall reverb more often than not. I like it better than my UAD Lexi 224 even. I think I'd probably like the 224 even more???
I can describe, in stupid, vague terms, the difference between the PCM-90 and any reverb plugin I own or have tried. But that's kind a pointless. I think a large part of that preference is familiarity too - I've been using the PCM-90 for a while now<G>!
I will also concede that the Exponential Audio plugins give the PCM-90 a real run for its money. As I learn more about both Phoenixverb and R4 I can imagine that I could retire the hardware.
Even more surprising, to me at least, Valhalla VintageVerb is up there on the list too. And for $50! Almost seems like it shouldn't be possible.
Which is where I start to split hairs -
The PCM-90 is simply the smoothest sounding reverb I own. I'm not saying it sounds like a chamber or a space, but it sounds like what I expect reverb in a recording to sound like, regardless of what I'm trying to accomplish. I just like the sound, and I can tweak it pretty quickly (never thought I'd say that when I first got it!)
The Exponential reverbs have a very similar character and sound, and any difference is likely my inexperience with them. I use them on individual tracks and stems a lot. (Of course they are plugins, so I can, that's gotta be a factor!)
The VintageVerb? It doesn't sound like any of the above, but it does sound remarkably cool, and I can use it on a track, stem, or even mix and get good results.
So I've taken to looking at it as a variety of tools at my disposal, and I'd hate to get rid of any of them.