Wireless is by its nature the worst possible choice for gaming since it has the highest latency.
Of course the best is an Ethernet cable and the second best is a pair of AV2-1000/1200 powerline adapters such as one of
THESE.
With wireless only you need to be very very close to the router/AP, not be competing with other wireless traffic, and not have any interference -- and then you will have a lot fewer problems with latency. The only time that I found the AC66U (which is a great router) sufficient for gaming otherwise was to use a pair of them with the second in media bridge mode and then plug the XBox into the bridge unit.