Most used paid for - Ozone 5 Advanced, specifically Insight (breaking them all apart).

Most used freebie - SPAN.

UAD LA2A "gray".  
 
I use a plethora of different plugs on every project but I just love the way this one sounds.

Could not possibly name just one.  But in terms of what I use over and over and over: Nomad Factory Pulsetec EQ, IKM 670 compressor emulation, Melda's MStereoSpread, FabFilter Pro Q 2.  There are probably a few others, but those are on the list for sure.

UAD Neve 1073 Pre-amp for recording
 
UAD or Waves SSL Channel Strip for mixing. 

Don't know if this was mentioned yet, but.....

 
Without this one, none of the others seem to work, so this would have to be my hands-down favorite.

......which reminds me of one of my favorite plugs..
 

i like psp oldtimer and use it on all tracks busses etc because it is simple to dial in but has a few coloration tricks too! 

The most consistently used for me is EZ Drummer, followed by Amplitube and Guitar Rig ... but you're prolly asking about audio processing. 
 
I'll go with the T-RackS Deluxe suite even though it's more than a single plug.
 
From within T-Racks, the Linear Phase EQ and the metering plugs.

#1 - Melodyne 4 Studio
 
#2 - S-O-M Plugin Rack
 

MXXX . It does almost everything. In fact it easier to list what it doesn't do than what it does.