This tiny little CEM3340 based oscillator is my #1 recommendation for someone looking into getting to Eurorack modular. It has all the key functionality and takes up little HP. And it can be useful to supplement a larger multi-voice patch.
The quality of Doepfer's knobs is perhaps the best in the industry. Their modules, including this one, are deep and won't fit some shallow skiffs. So check the depth!
Features I think are important for a good starter VCO that this VCO has. 1V/Oct tracking over a wide number of octaves (15 octaves according to Doepfer!). Exponential FM input, for 2OP-like FM sounds with another oscillator. Linear frequency modulation for vibrato effects. Soft and hard sync (I use hard sync mostly, so I have the jumper set there).
Or buy 2, and a noise generator (Doepfer A-118-2) and start making your own drum sounds.