The E5v4 Broadwell's are still the most powerful Xeons on offering. As Apple will offer up to 18-cores, I am guessing we're talking the 268x and 269x series so that would be the E5-2667 v4.
Intel is preparing to refresh the Xeons, well before the iMac Pro shipdate,
https://www.nextplatform.com/2017/05/04/intel-melds-xeon-e5-e7-skylake/