The first two Eurorack VCOs were the Intellijel Rubicon 2 and the Tip Top Z3000 mk2.
If I had to do it over again the Tip Top would have been an Intellijel Dixie II+. Nothing wrong with the Z3000. I would just prefer the interface on the Dixie after some wiggle time.
I would recommend a minimum of 2 VCOs so you can do sync as well as fattening your sound with two oscillators rather than one.