If you had asked the question a couple of years ago, I think I would have answered more or less anything made by Spectrasonics. Omnisphere, Trillian etc... But lately I've dropped most of the sample based stuff and gone for products like Arturias synths instead (hate their licensing system though).
 
/M

Rob Papen's Blue II.
 
Regards,
Dan
 

I'm not a big synth user - more of a sample guy - but when I need one I'll usually reach for Zebra. If I had the funds, I'd probably add Blue 2 to the roster.
 
Honorable mention to the free OBXD, whose sound reminds me of my old Oberheim and is just as easy to program.
 

Mach five and Kontakt normally my first synths - that's if we don't count drum programs like addictive drums. Then probably alchemy, Diversion from dimity sches then arturia / cakewalk stuff

Omnisphere and then Kontakt.

Still like Dimension Pro, Z3TA3, and Rapture - all of those have gems in them.

But Omnishere is the pinnacle.

For me it's the piano (lately Pianoteq and NI's electrics) and NI's B4 organ of course :-) Such a sweet Hammond. When I need a "synth" synth, I'll go w/NI or Atmosphere. 

Omnisphere
 
Alchemy
 
Tone 2 Saurus

Sylenth1. I think this was a game-changer in its day. I love the sound even though I don't record that kind of music as such. It upped everyone's game.

Omnisphere seems huge. Hard to explain until you have it.

Komplete, as a whole, is great, especially Kontakt.

Mesh
I suppose it all depends on what sounds you're after as each has its strengths.
I generally favour Camel Audio's Alchemy as my first choice. I do have a lot of content for it and, it's a very Phat sounding beast of a synth.....very inspirational.
I also like Z3ta+ a lot........and now, I'm starting to enjoy Synthmaster as well.
Korg's Legacy M1 is also very useful for me as well as Rapture.
There's just so many synths with NI's Komplete, so I don't even know where to start (Reaktor will keep you busy for at least 1 lifetime)........installing K9 as we speak.
 
Sometimes, I fire up a particular synth for a song I'm working on and then get completely distracted with that synth......forgetting my initial plan. 
 
 

 
This is exactly what I would have posted with the exception of Camel audio...I should look into that one..and Zebra.Hey Mesh my K9 is on the way :) I have been using Kontakt 5 for some time now.
 
Like Mesh says it all depends on what you're trying to do...I probably falsely group a lot of what I call "dance music" into one group, techno,electronic, trance...all that stuff uses synths with sounds you can get from Z3Ta+,Rapture  or a similar synth...many are big on arpeggios and patterns. I usually use synths more as an ambient sketch pad or backing music...and the thing is you can make similar sounds on many of these synths...a saw wave is a saw wave no matter if it's sampled or the real deal...it all seems to sound similar, just a matter of how you adjust it and the workflow of the GUI.
 
The more bread an butter stuff like pianos and organs are pretty much a staple in all of the more decent ones. If money is tight there are some pretty good either free or inexpensive ones out there. If you have Rapture, Z3ta+2, Dimension Pro...then you should be able to get started with most of it pretty well...unless you trying to make some really odd stuff...that would probably be Omnisphere.
 
I like the setup in Kontakt/Komplete...you have mostly anything you would ever need all in one interface and it's all easy to reach and play with...saves time in the studio. If I were moving on up from what came with Sonar,  Kontakt or Komplete would be my first choice. If you buy Komplete, you get Kontakt in the deal.

Congrats Tim on K9!! I finally upgraded and installed it last evening, but haven't had a chance to "play around" yet. I've been wanting to get Komplete for some time, but didn't have the funds when the upgrade prices came along.....until now. I'm totally excited for this weekend and looking forward to the buffet of sounds.
 
Good luck with K9......we should be set for some time!!