Then you can pretty much make a choice based upon the population indicated on the realm selection screen.
Higher pop means more AH activity and more players active in game. Note that a player community will be more active at some level ranges than at others. At this stage in the expansion, it will tend to be most active in the 110-120 level range.
Both the Chicago and the West Coast data centers have realms for all three main NA time zones. Which data center provides the best response time for you may depend upon your location and time of day/day of week.
wow.gamepedia.com can be searched for “realm list” to see timezone and data center info, including data center IP’s for running a traceroute.