The Korg Minilogue or Monologue seem like nice starters because they're inexpensive, featurepacked, and that little oscilloscope helps you understand how you're affecting the sound when you're fiddling with knobs and sliders. Monologue is a monosynth, so it's one note only, good for basslines. Minilogue has 4-note polyphony. If you had the patience and resources to save up a little more cash, I would recommend a Minilogue XD.
Another good bang-for-the-buck synth is the Behringer DeepMind.