Read this thread first.
https://forums.macrumors.com/threads/1597637/
Best is relatively to workload. If "best" is a euphemism for most expensive then either E5 2695 v2 or 2697 v2 Depends upon just how high Apple wants the max BTO price to go. If the '97 is super crazy priced Apple may stop at the '95.
For standard configs probably a E5 1660 v2 ( 6 cores 3.6GHz base 4.0GHz max ).