This soft synth is great for basslines. The resonance and filter are easy to mix with. If I'm just making new sounds, this one is easy. Its free!
Amplitube still has the best distortion simulations. It goes beyond the call of duty to model real life gear. It is the only thing suitable to mangle chords properly. Love this plugin.
I used to use Sylenth. I thought it was the best but I don't want to bother with it anymore. Looking back, I don't think it made my music better. I think its better as a second soft synth. I have a V-synth that covers all the same ground and then some.
This visualizer is free and its the best I've seen for visualizing the frequency spectrum. I need to use that a lot when mixing actually. I don't always trust my ears. Sometimes I know what a good mix looks like rather than sounds like.
Another great noise maker, but for beats. It seems to sound really good with Tal-Noisemaker without much EQ or mixing. You can mess up the knobs real quick. Its easier to use than Image Line plugins.
6. DIVA (updated March 2013)
I've messed with this before, and it wasn't as deep as I hoped, but it does sound perfect in the mix. It may be the only soft synth you need. make sure you have it on light CPU mode.
7. Sonnox Oxford (March 2013)
Not bad for dynamics and drum mixing. It doesn't color the mix much. I've been experimenting with it and it gives a high end sound.
8. iZotope Ozone (March 2013)
Still a good plugin with excellent presets. This may be the only mastering device needed.
That said, if I was going to buy one thing, I'd get the latest copy of Reason w/ extensions.
Other plugins you might want to look out for: TAL-DUB (Free), TAL CHORUS-60 (Free), Audio Damage Plugins, Addictive Drums, or maybe Rob Papen Albino (which seems to have cool arpeggiations)
Obviously you shouldn't take my opinion as something important. In the software world you can try everything that works for you.