Apples to make good bonsai.
They air layer fairly easily. Best to set up the air layer in late spring, so that you can cut it off before the end of August. This will give the new air layer time to harden off roots before winter sets in.
I have done a couple air layers of apples, several of commercial apple grafting understock. They air layer well.
Big Plus of taking an air layer from a tree that is flowering and fruiting - the air layer will bloom, and set fruit, just as soon as the air layer gains enough strength to flower, it will. Usually they begin blooming by the 3rd year after being air layered. An apple from seed might not flower until the tree is over 20 years old.
Personally, I would look for an interesting branch on a crab apple, the fruit are more in scale. The standard apples are good, and fine as a flowering bonsai, but the standard size fruit is so large that if you allow it to develop, you risk damage to the tree. The heavy fruit may break off the branch it is on, or the tree may weaken from putting all its energy into large fruit. But if you remove the fruit most years, and only allow one to develop when the tree is particularly vigorous, you can use standard apple trees as bonsai.
The comment was made that standard apples have leaves that are too large and branching that is too coarse. If you let the tree do what it wants, this will be true. See Morten Albek's Shohin Bonsai Blog,
http://shohinblog.com/ he shows photos of apples that have been trained as shohin. Notice his before and after images of defoliating to reduce leaf size. If you are consistent at applying technique, even standard apples can be made into Shohin (under 10 inches tall) bonsai. Of course it is easier to make a big bonsai from standard apples. IF you shoot for a tree that is 3 or 4 feet tall, the big fruit is not much of a problem.
Only real draw back of apples in general (including crab apples) is the fact that they are so disease prone. But if you are growing apples for fruit, you already have all the spray remedies you need, just be sure to hit your bonsai when you spray your backyard orchard. The same sprays you use on the field trees, will work, even at the same concentrations you use in the field.
Apple are good - I always encourage people to raise as bonsai edible species. EAT YOUR BONSAI.