I asked a similar question a while back and got a lot of helpful advice in this thread:
One word of caution: if I remember correctly, one of the popular suggestions was G935. However, it’s come out that G935 is susceptible to viruses, so if you are planning to multi graft, especially with traded scions, that may not be the best option. See this thread for more details:
Orange Pippin Trees also has a page that gives suggestions about appropriate rootstocks for different forms of espalier.