It depends. What sort of music are you into? If you like funk and want to play weedly weedly bass sounds, you buy a Moog monosynth and you're set. If you want to play trance or dance styles then you figure out arpeggiators and sequencers and get to know the synth architecture so well that you can program it on the fly, because many styles of music are about taking a simple repeating motif and redoing it in different tonal colors. If you want to play synths as backing instruments in a rock or pop combo buy a workstation keyboard from Korg or Yamaha.
Tell me what you like to listen to and I'll recommend you something.
Tell me what you like to listen to and I'll recommend you something.