I recommend Absynth by Native Instruments or Rob Pappen's Blue or Albino 3 . These are excellent for Trance , they have powerful pads , also Rapture would be a good one for leads , but i like to tweak them or make my own either way
You should look in to dynamics and timebased signal processors more than collecting an ass ton of synths. The synth is just one part in a long chain of gear that produce a lead/bass/pad line that make a solid. A lot of the top names in the industry are still using the same synths they have been for years because a larger part of the sound is what you do with the signal once it leaves the synths outputs.
Just a thought.
I'd give you a list of dynamic and timebased software plugins, but I use protools which is RTAS and TDM plugins, and I do not know which ones are also VST.
Didn't see anyone mention Vanguard so I'll go ahead and throw that into the pot. I use it in nearly every production and have done quite well with it over the years.