The best dwarfing rootstock for your area depends on your location, soil type, and what apple production system you wish to utilize. Your location determines likely cold hardiness, vigor, and disease issues. The soil type will influence tree vigor and ultimate tree size. Your apple production system determines whether you need a rootstock which will result in a free-standing tree and what size that tree needs to be, or whether the rootstock needs to be more dwarfing and will result in a smaller tree that will be supported by a stake or trellis. When you have a candidate list, it is probably a good idea to talk with commercial nurseries selling apple trees to see which among your candidate rootstocks are available, and which have the apple varieties you wish grafted to them.
See the Apples resource area on extension.org for more information about dwarfing apple rootstocks.
Answer provided by Robert Crassweller, Penn State University.