My vote for NI Massive for its feedback loop and frequency shifter. And Synful, that I don’t own yet for accurate classical orchestra emulation.
Just wanted to add another notable mention because if it didn’t exist, my worklfow, complex as it already is, would be even more fragile.
Not an effect, more of a utility: ToneBooster’s EBU Loudness Meter. Purchase entry is pretty much free but I opted to support it,
because 1) there’s currently not that many available and 2) its pretty cheap or free if you want that route compared to other$.
I gotta say a synth made by a fellow Renoiser called EFTAHigh! CRAZY!
I couldn’t choose one. I’ll repeat the love for Massive and FM8 and add Arturia Minimoog V, Sonic Charge Microtonic, D16s drum machines and Arturia Jupiter 8.
For FX, well, the renoise fx that let down the most IMO are eq, compression and reverb…
For EQ I use Gliss EQ and URS Channel Strip Pro
Compression: PSP Oldtimer and URS CHannel Strip Pro
Reverb: Toraverb and Sony Oxford.
I own a ton of plugins, more than I should I suppose. Spent way more money than I’ve made. Thank god for day jobs huh!
Anybody used Stillwell Audio’s (Schwa’s) Olga and if so what is your opinion?
Used it briefly when my desktop was still up and running and thought it seemed to have quite a good sound, even if a little CPU hungry if you decide to push it. Managed to get some powerful sounds out of it playing with the Unison modes.
On the whole I like Stillwell Audio’s policy for licensing, even if they may not quite be right down at what I consider Budget price they are still a lot more affordable than most commercial products and none (very few?) in that category have a evaluation policy as open as this as far as I know.
“Please feel free to download evaluation copies of all of our plugins. The evaluation copy has exactly the same high-quality processing as the licensed version, no beeps, hisses or odd silences. Give them a try – we think you will want to continue to use them and we hope that you will agree.”
Windows and OSX, just need to persuade them to port to Linux as well now 
Olga is really good but you only really reach for it if you want something to sound half destroyed. It’s pretty much perfect for that.
I’m considering going for a compact setup, Loomer Aspect VST and Arturia’s minibrute hardware synth.
I’d love a minibrute.
What if you want something fully destroyed? 
Synth1, INTRO, MiniMogueVA, NoiseMaker, …there are so many amazing free VSTi out there!
Buy Windows Me.
My favorite VST/synth would be monstrumWave/Waldorf Microwave XT
I designed and programmed monstrumWave last year in ctrlr along with a few other panels for a few other synths. If you aren’t in the know, ctrlr is a Juce based application that allows you to create VST panels for your synths which work in most every DAW (although renoise has some issues as VSTs in renoise don’t speak MIDI… yet).
Many refer to the MicroWave XT as a “knob monster”, but what most don’t realise is that the knobs on the XT represent a relatively small fraction of the 250+ parameters within the synth. This panel brings things like the extensive modulation matrix (and literally every other “hidden” parameter) up to the same level for better accessibility and usability. I use this panel heavily with the XT and recommend ctrlr to anyone who isn’t afraid of decoding sysex messages and writing Lua code to custom build functionality.
Here’s a little demonstration video I made to show it in action. I’ve updated a lot since I made this video though, all of this is available for free (besides the synth) on sourceforge.
/shameless self promotion
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.