Theoretically for ETH the best GPU would be the Polaris series (RX470/480/570/580) but they are in limited stock right now. If the prices aren't inflated GTX 1070 are great for Equihash mining. GTX 1060 and GTX 1050Ti are great for any GPU algorithm in general. Just buy any GPU that does not have its price inflated, you can actually mix GPUs of different models and even between AMD and Nvidia (others did it before) but there might be some driver issues if you do that.