The east. Russia and Poland look like they should slow the Horde down so I don't have to worry about them, right? :D
if only that were the case... Russia looks like a paper tiger. She looks scary but i doubt there's much bite in those fangs. Usually if Byzantium is alive they stop the horde in their tracks because they can match their numbers. The Rus normally don't have those numbers unless they pour hundreds of thousands into the steppes like I did. And I still lost!

If i had to lay odds I'd say the HRE would stop the horde - but even then it's gonna be rough.