I'm very very very new to mining so take my recommendations with a pinch of salt, I'm sure some guru's would correct me if I'm wrong.

To start off with:
  (a) AMD RX 4XX/5XX range are pretty good for dual mining. Personally I went with PowerColor Red Devil Gold Sample RX 580 8GB cards.
  (b) nVidia GTX 1060/1070 range are pretty good too. Personally I went with Zotac AMP! Extreme 1070 TI cards

Both of the above cards were evaluated on the BitsBeTrippin youtube channel if you want to pop over there and see what they are capable of.

I guess it all boils down to your budget and how many you want to run. The AMD RX Vega's seem to be kicking ass with CryptoNight algorythm too.