What a wonderful idea!
The "best" kind of tree is "any" tree in my opinion. Are you looking for a long-lived species such as swamp white oak; a fast growing species like yellow-poplar; a spectacular display of fall foliage such as sassafras; or a unique conifer such as dawn redwood? Your options are virtually endless. However, I would discourage you from buying any ash trees!
Bareroot seedlings may run you anywhere from $0.40-2.50/tree – these figures are dependent upon species, number ordered, and size. 3-gallon containerized stock may run $8.00-30.00/tree.
Please visit http://www.mortonarb.org/plantinfo/plantclinic/planttrees/plantclinic_planttrees.htm for more information and images on desirable Midwestern tree species. Again, “best” is merely a matter of interpretation :-)