The best grafting material is the one in the best shape. Try to get them in febuary, seal the ends, put them in the fridge in a bag. I wouldn't add any water to the bag, like a moist towel. A commercial tree guy told me he wipes his scions in a light chlorine water mix to get rid of mold issues.
Far as what crabapples, crabaaples general leaf out and bloom very early. I'd be eyeing bloom groups.
Here in several local orchards, instead of crabapples to promote polination, they mix in golden delicious trees. Some people consider fraklin cider to be a crabapple tree. Its some fort of frankenstein seedling from a orchard in VT. However, it's a bloom group 4 tree, october ripe / drop, zone 3 friendly, seems to make good fresh or hard cider too. galarina is a smaller apple, late droping, bloom group 3, and has extended bloom period like golden delcious does. Golden delicious is my only cedar apple rust exception.
Far as crabapples for strickly deep, the whitetail crab ones are good. crossbow is iinsect resistant and not too early to wake up. droptine's a good one. AWHO and winter wildlife from saint lawrence nursery are good too. MY droptine and 30-06 on anty survived a -35 deg spell up north last winter. Thinking winter wildlife will be a good one for me.
Why pick a single variety. Graft a few different ones. Any scion I get for rootstock I also put on mature trees. 2 or 3 extra buds on a stck can make 2 to 4 scions for more rootstock next year.