Flixbus provides daily more than 10,000 connections throughout Europe! Check the whole route network on FlixBus.de and travel save, comfortable and substainable. Enjoy the free W-LAN, cheap snacks and drinks as well as legroom and enough space for your luggage (2 suitcases + hand luggage).
As an owner of the ESNcard it even become cheaper. Log in on http://esncard.org and get a 3 EUR Voucher.