xiwix

arachnaut
I think I use Reaktor every day.
 

I'll second NI Reaktor.  I've got like 10 instances of Reaktor in my favorite template.  Compression, effects, drum machine, synth.  

I would think if you could reach a high level of knowledge of Reaktor you would buy less synths.

MCompare - This is one of the few plugins I can say actually improved my mixes immediately and saved me time.

S-gear - although the new version is taking a long time, I love this software and the tones I'm able to get out of it. It's the only gtr sim I use.

MXXX - it does everything, so of course it's on here.

Slate's VMR, for sure. I haven't mixed a track without since I got it. I don't use every single module in it, but most of them are indispensable for me now. I subscribe to the Everything Bundle, so I use a ton of Slate stuff all over my mixes, but if I had to choose, VMR would be my desert island plugin. 

 . . . when you have the best, you tend to use them regularly
Fabfilter Pro-Q2
Fabfilter Pro-C2
BFD3
Melodyne Editor4
Ozone 7
 
. . . wait, the thread title said we have to pick just one ?
Okay, BFD3

Robbyk
 
100% agree! I completely forgot my TrackS plugs! I master all my projects with them and I use them on many channels especially vocals!
 
I can't believe I forgot to mention them! Doh!

Hmm... Tough one...
 
But in order of usability (for me that is):
ARC 2
Ozone 7
Omnisphere 2
Alloy 2 (I could use Ozone in stead, but Alloy is just lighter on my system).
 
I mostly use what is "in DAW" (I use Sonar, Cubase and Live.. each depending on the situation and location), but the above plugins are always in my "toolbox".

Kontakt.

Well, if we're including Kontakt, then I'd have to say Kontakt as well.

Revoice Pro

Purely because it turns hours of manually syncing twin tracked vocals into minutes.

Royal Yaksman
Revoice Pro

Purely because it turns hours of manually syncing twin tracked vocals into minutes.

This is going to make One King Larry very happy.