If you are gaming in Ultra at 1440p/~60fps *or* 1080p/~100fps++, then, get a GTX 1070.
If you are gaming in Ultra using a G-Sync monitor at 1080p/~60fps or less, then, get a GTX 1060 (6GB)
If you are gaming in Ultra using a FreeSync monitor at 1080p/~60fps or less, then, get an RX 580.
If you are gaming in Ultra with a non-G-Sync/non-FreeSync monitor at 1080p/~60fps or less, then get either the GTX 1060 or RX 580 whichever is cheaper.