Surge XT

Surge XT is the best free synthesizer VST plugin right now. If I could only recommend one synth from this list, this would be it. It is a hybrid synth with a dual synthesis engine, three oscillators per voice, eight algorithms per oscillator, two multi-mode filters with eight filter types, twelve LFO modules per voice, eight FX slots with ten different effects, and more. The oscillators cover classic analog waveforms, frequency modulation, and wavetable synthesis.

Surge XT can even process audio input as the sound source, turning it into a versatile audio effect. A modulation matrix and a preset manager for storing and editing custom sounds are included as well. The interface is fully resizable, and the plugin has consistently improved with bug fixes and new features since it became open source. The programmers update it almost daily, and you can access nightly builds if you want to stay on the bleeding edge.

Surge wasn’t always freeware. Vember Audio originally priced it at 99 euros when they announced it in 2006. It became open-source in September 2018, and the Surge Synth Team (the same open-source team behind the OB-Xf synth and ShortCircuit XT) has been driving development since.