This… Alchemy is my favorite too, followed by FM8 and Absynth 5.
DIVA! i truely adore her 
It’s probably useful to know what type of music these plugins are used for. The commercial VSTi’s I really like are Z3TA, Toxic Biohazard, Zebra and Sylenth. That’s mostly for dance/lounge/downtempo, they’re not that great for dubstep or things like that (I think). Don’t underestimate the freeware VST’s though. Lately I’ve been trying to make everything using nothing but freeware, just for the fun of it. TAL-Noisemaker, Oatmeal, Voyager, SQ8L, synth1, or even just mda DX/JX10 are really great for certain sounds. They usually require a few more effects and tweaking to get the most out of them, but if you got more time than money, that’s a plus
Instruments that i mostly use is synth1, v-station and garritan personal orchestra. Those ones i can’t do without. If i need to choose only one, it would be synth1 because it’s so versitile, powerful and light.
I have just one commercial synth - D16 Phoscyon, because I couldn’t find any decent acid bass synth a year ago. And am lovin’ it… also using TAL synth line most of the time, U-he Tyrell N-6, Zoyd and Zebrallete, Synth1, Automat, Audjoo Helix from time to time (latest freeware version) and some other odd ones.
Plogue’s Chipsounds, surprised no one has mentioned it yet.
I’m glad someone took the helm to not only save the synthesis techniques of days past, but for some, including me, childhood memories.
Its pretty amazing, to participate and almost reformat, recompose some of those memories.
For me it’s definitely SQ8L. I know it’s kind of an odd choice, but I just love the way it sounds. It makes its way onto so many of my tracks. A close second would probably be Helix, though it’s GUI often turns black for some reason and I can see anything to change my patch, so I end up stuck with it for that song. 
I’ve been really enjoying Oatmeal lately. I also really enjoy the TAL line, Phoscyon, and QuadraSID.
Talking of this I love QuadraSID to an almost unhealthy degree.
Great thread. I also started looking for some VST/AU to buy recently, and have been evaluating a few different ones in the last few days, and there are probably a few in this thread I want to look at as well. The one I’m currently most impressed with is Aalto. I just wish it had a Linux version too, that would make me buy without further hesitation.
My number one requirement is “no DRM”. Luckily Aalto was one of the few interesting synths that did have reasonable (no) copy protection, so they might get my $99.
I like oatmeal because of the random option, randomize until I find something interesting then maybe tweak some parameters, turn off the synths fx and render the plugin to a sampled instrument for pattern command + native dsp action = win.
Recently I have been using this suite a lot: http://www.genuinesoundware.com/?a=showproduct&b=37
Here’s a comparison of his Rhode VSTi compared to the real thing:
I liked the sound of this plugin untill I noticed how bad are the higher notes and how much DC is added to the signal. I suggest you take a closer look at it.
Some I like: Synth1, the rest in no particular order: Invector_8v, Rez, Texture 1.2, iblit, Triangle II, Freehand Free, oatmeal, LoganA.
(Yeah, all free stuff.)
Edit again… I’ve edited this post like a lot… lol.
Hey, that Arturia jupiter rocks!
btw, for what its worth, this thread totally inspired me to clean out the old, “vstplugins,” folder… yeah… I just ditched a ton of freeware stuff ( but not all of it ) some freeware is pretty darn good.
just deleted a lot of demos… ** but sooner or later, I’d like to purchase that Arturia Jupiter… lol… it totally rules. beautiful low end imo
+1 for the Diva. When i get some cash she´s my nr. 1 purchase.
Harmor.
i mostly use medusa2 (free) and dirty harry ($15).
other freebies:
transister organs: comboV, comboF
‘chip’: unknown64, 38911 bytes, basic_64, tweakbench synths, famisynth
mellotron: tapeworm, meltron
maybe the ‘best sounding free synth’ is zebralette; also, it’s reasonable on the cpu. u-he allowing people to get a taste of the oscillator sure is a great marketing ploy to sell zebra…
Alchemy and Reaktor.