Apricot

This is one of the best free synthesizers that I've tinkered around with. I'd say it's mostly ideal for tracks in the trance umbrella, because of the factory preset library (one of the best I've heard, including commercial synthesizers) and the unison per main oscillators being modeled off of the JP8000 supersaw.

The workflow of this synth is also really intuitive, everything is clearly laid out and quality patches can be dialed in with minimal effort. This includes the effects section, where adequate modulation and spatialisation can be done with little tweaking. This also applies to the arpeggiator / sequencer, where even if they're a little basic, great results can be made from them. Really nice for the price of nothing.