Naumburg Cathedral is clearly not the first cathedral on the list and it's a fair guess that it won't be the last, but I have to say it is one of the nicest ones - by which I mean that it is really well-preserved, with hardly any post-medieval alterations, and gives the look and feel of an authentic church from the Middle Ages. Its four towers can be seen from far away, and its interior is full of fine late Romance and early Gothic archiecture, especially the works by the so-called Naumburg Master. There is also a very nice cloister and a treasury. Located in Martin Luther's homeland, it is also notable for being the first Protestant cathedral and diocese in the world, and the first Protestant bishop anywhere was personally installed in the cathedral by Luther himself (it remained a diocese for only about 20 years, though). I reached Naumburg by train in a half-day trip from Leipzig.