Etherium is a new cryptocurrency. As with any cryptocurrency it is profitable at the start for a while. If you can get into the base of the inverted triangle before it grows too large, you can make some great returns, at the expense of the people who show up later. But of course since it's an inverted triangle, it's not a pyramid scheme, so don't worry.
The "best" is pretty much the best you can afford. Whether that is an R9 295X or a GTX 1080Ti, or an RX460 or a GTX 960.