Surge XT is a must. Best FOSS synth there is IMO. 3 oscillators in 2 scenes. Filters, effects, all the LFOs and envelopes in the world, all the modulation, expression aftertouch, etc you need. A bunch of presets out of the box. Very flexible synth, though can be a bit learning curve to get going.
Honorable mentions to Dexed (basically a software DX7), GeonKick (for synthesizing drums), and pianoteq (proprietary, but best there is in piano synth with native linux support).