Most commercially produced apple rootstock are from random apple seeds. In the United States, random translates into Red Delicious.
Red Delicious seeds do not produce bad rootstock. They tend to be fairly resistant to fire blight and fairly well anchored. In general, they lack winter hardiness in the coldest regions and they are vulnerable to wet feet.
All commercial apple orchards are now planted on dwarf and semi-dwarf rootstock. The time value of money requires that revenue be generated as soon as possible to start servicing the debt. That debt can run from $60k to $100k per acre. Just to provide a frame of reference, a standard high school football field is about 1.3 acres.
Much research was done on sources for seedling rootstocks prior to the 1940s and the "modern" emphasis on dwarfing rootstocks.
Of the common varieties, McIntosh was found to have the best anchorage (although more vulnerable to wet feet than Delicious).
A minor variety called Mercer was touted by some as a good parent. One shortcoming of the older research is that the researchers did not always tell the reader what their selection criteria were. All I can tel you is that some writers were fond of Sweet Bough X Mercer but I cannot tell you why they thought this was a good set of parents.
Hardy crabapples from the Malus baccata, Malus X robusta and Malus prunifolia cluster of species were often used in the most extreme climates....like Saskatchewan and the Dakotas, Russia and China. Growers in these regions also like varieties with these species in their pedigrees: Kerr, Dolgo, Columbia, Robin, Bedford, Ranekta and so on.
Incidentally, some researchers (like Forsline) are dubious about Malus prunifolia being a valid species. They look at the variation within the "species" and think it looks like a pile of the misfit toys got swept into...much like you would expect to see in a flock of bastard children from M. baccata and M. domestica.
There are a few niche sources of apple seeds. For example, Lawyer Nursery lists "Columbia crab"seeds in their seed catalog. Columbia crab is one of those Alaska/Sasketchewan capable varieties. They also list M. baccata and M. prunifolia seeds.
|Liberty fruit as photographed September 1, 2015 in Eaton Rapids, Michigan. As always, you can click on the pictures to embiggen.|
|Seeds from Liberty. Extracted September 1, 2015. Not quite ripe.|
|Ben Trio. These fruit are about the size of golf balls. Photo taken September 1, 2015.|
|And you would be wrong. Seeds extracted and picture taken September 1, 2015.|
The best M x robusta I have is this one selected for having fruit that hangs on all winter long. One potential issue with crabapples is that they are often sensitivity to apple viruses. Modern rootstock breeders now believe that many cases of reported "incompatibility" were actually due to viruses in the scion wood killing the rootstock. The baccata-robusta-prunifolia corner of appledom is not the most sensitive group of crabapples to viruses but cases of "incompatibility" have been recorded with Malus baccata.
Composite picture showing relative size and color of the seeds
|Upper-left, Malus x robusta. Upper-right, Ben Trio. Lower-left, Liberty. Obviously, bigger seeds can be expected to have greater seedling vigor when they first start growing.|