Depends, what are you planning to mine?

(All the rates i'm posting are stock and not overclocked)

If you're going to mine ETH, the best bang for buck would be the AMD RX 580 - it gives you around 22 mh/s, your next best bet is the NVIDIA 1070 - gives around 27 mh/s.

I would recommend you go with around 4x RX 580's (don't forget you still need a MOBO, PSU, GPU risers, etc), so take that into account.

I would personally go for 4x or even 8x rx 580's.