I would choose a Chinese elm Ulmus parvifolia which is listed as a zone 5 tree according to my Encyclopedia of Trees by Hugh Johnson. They have small leaves and do well in pots. I have several of them as bonsai in my zone 9 area. Good luck.

The hardiest elm is the one growing in your backyard! Seriously, nothing can compare to using a native tree in your area in terms of cold/heat tolerance. There are a lot of native elms in the NE region. Start with a seedling and it'll grow faster than you think! Along the way you'll be able to learn the ins/outs of working with the species.

I don't know if I could recognize an elm seedling but there a few people working on native American elm resistant varieties.
In New Hampshire there is the liberty Elm and there are others.
I'm hoping someone can tell me (us) if these resistant varieties are available to the public and where we can get a hold of them.
Paul

According to Bonsai4Me, an elm bonsai would not be subjected to the disease due to its size and should it be afflicted, could be easily cured. They also have photos of Elm leaves... just look for the serrated edges on the alternating leaves, as well as the uneven base of the leaf (one side is further down the petiole and closer to the branch than the other side).
http://www.bonsai4me.com/SpeciesGuide/Ulmus.html

However, if you would rather have a cultivar designed for resistance try looking for any of these:
http://www.extension.umn.edu/yardandgarden/ygbriefs/p425dutchelm-resistant.html

Many, if not most of the volunteer elms growing in fields & on ditch banks in your area that are thought to be U parvifolia are actually U pumila, a species hardy to USDA zones 3 & 4 and much more cold tolerant than U parvifolia ...... if you can deal with its random twig and sometimes branch dieback.

Al