My recommendation would be a Vega 56/64, although most of the AMD card prices are inflated right now (holidays? shortage due to CNY? I have no idea why, but maybe try Ebay). The RX 480/580 line is apparently going unsupported in ROCm, which is quite sad, but the Vega line is known to be well supported by all of ROCm, while not being absurdly priced.
That said, I’m personally looking forward to when the RX 6800 comes down in price (and ROCm support gets better), because I’d love to play around with the raytracing instructions!