I have too many instrument plugins, like anyone else who uses a DAW. Some are free, some are paid, and put them all together and ya got options paralysis. So I decided to get out the pruning shears.
Here's what I consider the top plugin instruments, the ones I can't do without:
Reaktor. Duh. It can do things nothing else can do and I don't see that changing anytime soon.
Synthmaster - this thing has completely spoiled me for most other "straight" plugins. And by straight plugs, I mean not a construction kit like Reaktor or an elaborate scripted sampler like Kontakt. I can load up a beat and noodle around on Synthmaster presets for hours. It's like ear candy laced with ear crack. Ridiculously gorgeous sounding. If you think the buzz about Synthmaster is just hype - nope. Try it and see for yourself.
Geist - a superb plugin for cranking out swinging MPC type beats. Well designed, great workflow, enough bells and whistles but not too many; it's a productive and stable beat-making environment with low to moderate resource usage.
Tremor - for a drum machine from the same company that makes Geist, this has very little overlap. The polyrhythmic and probabilistic features remind me of my own ParamDrum ensemble for Reaktor, but it uses synthesis instead of samples. Going by what people say, I think the sound is a love or hate thing. I love it to death, me. Great workflow like Geist. I can put together a beat in this that morphs over time and just listen to it chug hypnotically for half an hour instead of listening to an album.
Kontakt - like Reaktor this can do things nothing else can do, and approaches much of what Reaktor does in the area of sampling, thanks to its scripting language. I find myself exploring Kontakt more and more, and using it for creative sampling rather than as the big dumb multi-gigabyte library playback leviathan it also is.
Honorable mention: DCAM Synth Squad. The mind melting sound is unparalleled in the sphere of analog emulation, but the lack of a stable 64 bit version has me reaching for it less these days. I hope the FXpansion fellas do something about this soon, so I can add it to my list of vital desert island coconuts.