What's your single most useful plugin at any price?!

While current favorites wax and wane...Waves Butch Vig Vocals being the current headliner...the UAD LA2A grey model is still the most reached for.

Not a plugin in my case as it runs on dedicated hardware, but Dirac Live. They do have a plugin version of it though so I'm counting it :D.

Ozone 7.  Always does the business.
 
Seeing FabFilter mentioned, I used to be a big fan until I found they really slowed my system down, even though I have a fairly good PC spec. For some reason they are the most memory-inefficient of all plugins I own.

FabFilter Pro-Q2

Channel eq. Followed by spectrum/correlation analysers such as Span and a few other "utility" plugins.

I could, at a push, do pretty much everything in hardware, but it would be slow, painful and generally impossible to get exactly the same settings twice. A hardware eq capable of the kind of very narrow, accurately located cuts and boosts Sonar or Logic's channel eqs can do is something I can't do in hardware at all easily or repeatably.

In "effects" plugins I guess I use reverb and compression most. The reverb tends to be Valhalla at the moment, and the compressors that see most use are 1176 and LA-2A types. Cakewalk provide good examples of both, I marginally prefer the Waves 1176, but the CA-2A is a bit better than the Waves' version to my ears.

Oh zone. Oh yeah.

Omnisphere 2 VST
 
close 2nd is Maschine 2 VST
 
only ones that keep inspiring me even when I am at my lowest, these two can get me out of a funk, inspire or get the creative juices flowing again.
 
if your talking effects then I would say SoundToys 5 bundle with the new rack.

Over the years in nearly every project I can say that I probably have
 
UAD EMT140
UAD Helios
UAD SSL or Waves SSL
FabFilter ProQ/Q2
FabFilter Pro-L
Bill Wall's LimitX
Slate FG-X or Ozone 5
 
Newer notables (becoming more go to)
 
BOZ Manic Compressor
Ozone 7 Advanced
Waves Greg Wells Voice Centric
Waves Abbey Road Plates
UAD 1073 (latest version)
UAD 88RS (latest Version)
Slate VMR
Slate VTM
 
 
I apologize for not noting just one. I do not believe that I could get by with just one.
 
OK, maybe Ozone 7 or Slate FG-X bu that depends on the style of song (for me at least).
 

Gotta throw some love towards Slate too. His VMR is getting better and better, now including those famous bluey 1176 compressors.

SONAR's own QuadCurve EQ.    I haven't had a single project without it since first introduced.
 

CW's CA-2A, Arc 2, and Wave's L1 limiter.

A year ago I would have said Arc 2, but not Sonarworks for sure. 
 
I also use HornetVU meter on every project. However, once I use it on each track to figure out the trim adjustment I make the adjustment in Sonar and delete the plugin.
 
EDIT: BFD3 and Stylus are in every project I do. I was thinking only effects and not instruments. duh. :)
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I also seem to reach for the UAD Pultec and UAD 1176 a lot. And the UAD Ampex on the master buss almost always. And the UAD Fairchild on drums most of the time.

My most used plug-in is probably my most boring. And it is only the most used because I have it inserted as part of default track template.
 
Breverb 2
 
I'll get me coat...