Currently for new GPUs, best ROI is still with RX570/580 8GB that can be bought bulk for around 110USD per GPU depending on the quantity.  This is what the majority of our BIG BULK (10k GPU or more) customers go for when ordering Octominer rigs.

Right now there are also very good deals for Nvidia P104 8GB GPUs, that perform very well on Ravencoin, Beam and Ethereum also. ROI is around 300 days at current prices at 5C electricity.