Kontakt 5, is where I usually begin.

If we're talking about virtual instrument, EXS-24, my sampler. By a long, long shot.
 
Otherwise:
Steinberg's Retrologue
Logic's ES-2
Cakewalk's Z3TA
All the Korg synths

Hardware: Korg Oasys 76
Software:  Spectrasonics Omnisphere

Kontakt 5 - especially some of the crazier HS libraries.
 
Absynth, Rapture, Dimension Pro usually end up in a track.
 
Alchemy has some good sounds, but I haven't really mastered it yet.
 
Razor and Monark are fun, but I haven't used them that much yet.
 
On occasion I also find use for Rhino, Wusik, Pentagon, Crystal, and the AAS Player libraries for the String Studio.  I used to use the Ugo stuff a bit, perhaps it is time to revisit those.
 
 
 
 
 
 
 
 
 
 
 
 
 

The Delay Lama

dubdisciple
The Delay Lama

Yes, right, I used to be infamous for using that one.

Most of my projects include one or more instances of Rapture. It's a very versatile softsynth.
 

Seems like lots of synthmaster lover here. Count me in. Also like Zeta+2 and Kontakt. 

Well, I left Kontakt out because I do not really class it as a Synth, but as others have included it I will say that this is my most used VSTi by a big margin, that said I don't use the factory library that much. Mostly other NI and third party stuff.

mumpcake

dubdisciple
The Delay Lama

Yes, right, I used to be infamous for using that one.

Never got to use it in a track. It probably would have worked in a dubstep track but it died before the genre took root.