In general GTX 1060 slightly outperforms RX 480 on today's games. However, RX 480 8GB outperform GTX 1060s more often on DX 12 games, so as time moves forward that will be the better card.
Also, there are some titles which are extremely VRAM intensive, for example: Skyrim. Double that if its modded Skyrim. Triple it if the mods are texture based. There are a few other titles like that. In these cases, any card with 8 GB of VRAM will outperform any card with 4 or 6 GB of VRAM. And of course any card with 12 GB of VRAM will outperform those in turn, if anyone wants to spend that sort of cash.
AMD started with bad drivers on the RX series, but the latest 2 versions are working well and they have finally caught up.