why does hydrogen have the greatest migrating aptitude!!??

The key to the migration is a donation of electrons to the other oxygen atom. If a RCH(OH)OOR" is created from an aldehyde, then the apparent migrating group is a hydrogen. I think of it as simply a loss of a proton and the product is carboxylic acid rather than an ester. If RR'(OH)OOR" is created from a ketone, then the group best able to donate electrons migrates. Here, it may be helpful to draw the intermediate as an -O(+). Then you may think about which atom can best donate electrons to it, namely a tertiary carbon is better than secondary, primary, or methyl.