Best Ethereum mining GPU [Sep 20th, 2016]
Currently, as of September 20th 2016, the best GPU for mining Ethereum is the AMD RX 480. The criteria are cost, hashrate (in MH/s) and power usage.
4GB or 8GB?
4GB models of the RX 480 offer a lower initial cost, but resale value will likely suffer. Memory in the 4GB models is also generally slower.
8GB models of the RX 480 offer additional future proofing and should resell better.
Second choice is the Sapphire Radeon NITRO+ RX 470 8GB card for approx 22-25MH/s. However, please be aware that in addition to the lower hash rate there is also less of a support community for the RX 470 cards IMHO. 🙁
What about NVIDIA? None of the current gen NVIDIA cards (GTX 1080, GTX 1070 and GTX1060) are cost effective for mining. If you happen to have some of the GPUs around for gaming then feel free to use them for mining… they just do not offer quite the bang for the buck as the current AMD Polaris lineup.