I'd suggest an analogue monosynth, preferably with plenty of knobs and sliders. Software synths and digital hardware synths are both programs running on computers, so unless the hardware has a distinctive sound unique to that manufacturer (e.g. Waldorf) might well not be that different in sound quality.
As for which synth, any Moog, the MS20-mini or Korg ARP reissue, Waldorf Pulse 2 (uses a control matrix but it's a good and easy to understand one), Arturia Mini or MicroBrute or something like a Doepfer Dark Energy/Dark Time combination. Or even take a look at the Korg Volca range. They're cheap and not exactly the highest quality components, but the bass and drums ones in particular sound pretty good.
I also suggest looking for a synth that has MIDI ability for at least note pitch for convenience, but functions as a voltage controlled synth. Voltage control makes smooth pitch and filter sweeps possible without the stepping effect MIDI often causes.