AudioKit Synth One - Free & Open-source iPhone/iPad Synth 🎹
Omnisphere - Piano, pad and synth VST for DAW's.
Sampulator - Make (and record) beats on your keyboard
Serum - VST for FL Studio, Ableton Live, and many other VST supported DAWs. Heavily utilized in EDM.
Logic Pro X - Music production.
Korg Legacy Collection - Korg Legacy Collection is a suite of VST instruments that acts as an audio plugin software for integrating synthesizers and effects units into your digital audio workstation.