I nominate the Rega Exact 2. It's a dead-nuts compliance/resonance/alignment match for an RB300, price is in your ballpark, doesn't require anything fancy in the way of stepup transformer or hi-gain preamp, and in its element (i.e., on a Rega tonearm), is fast, open, relaxed, and very natural sounding.
Ortofon 2M Bronze or Black also.
Ortofon 2M Bronze or Black also.