this would be just ONE go-to plugin, 
that you use the most,
or
is simply your favorite..........
 
 
i'll start:
 
Waves L-1

I'd have to go with Izotope Alloy, it was my first really nice plugin long ago and I learned a lot on it. I still use it but also supplement with Nectar.

Izotope Ozone.  I have version 4 which is old.  I think they are on 7 now...

i have been slapping a cautionary WaveArts FinalPlug on my master out for over 10 years!

If I had to choose just one...
Ozone 7 Advanced...
it's the Swiss Army Knife of plugins...

I simply cannot answer that question because my favorite non-CW plugins serve such different purposes.  But I can tell you the three that I probably use the most:
- Melodyne 4
- MJUC compressor  (tracks and master)
- Ozone 5 (master)
 
I'm also assuming you meant FX only, 'cause instruments are a whole other ballgame...    
 

Melodyne 4 Studio
 

so, 
when you guys say 'ozone',
isn't that a suite of plugs?
 
i'm looking for a single plug...
such as 'maximizer', or 'dynamic eq', 
that kind of thing.
 
 

Two way tie for "m"astering:
Ozone7/Slate FG-X because neither is perfect for EVERYTHING.
 
Heads above all else for my bass tracks: UAD Helios.
 
Best all around Reverb is a four way tie:
UAD EMT 140
LXP-480 Complete
PSP 2445
QL Spaces
 
Everything else I own from UAD/Waves/Slate are in a firm 2nd place.
 
 

Izotope - Nectar and Ozone