As always, the best way to take a look outside at the world of hostels in Europe is not to listen to what their owners say, and even I would say it is not about looking at the punctuation that they have in travel pages. From my point of view, there is no better source than looking at what users from Interrail or Eurail say after being in some of this hostels. I have done some research on different Interrail local forums and here it comes a selection of the best hostels in Europe.
For Germany, if you are looking to travel to Hamburg, a good recommendation is Auf Dem Stinfang. clean and tidy according to the guests, good atmosphere and the breakfast simply incredible. Going now to the capital, the best pick is the Eastern Hostel, specially because it is further from conventional. It is in fact a docked boat, so the experience of sleeping in a floating hostel seems incredible. Quality on board is very high for being just a hostel and many of the rooms are underwater, which can sound like exciting or scary. It all depends on you.
If you go past the border and enter in Poland, you will need to visit Krakow for sure. The best place where to spend your nights will be in this case the hostel called Giraffe. Clean, brand new, very fashion, and apparently with some very kind and good looking girls at the front desk, make it a combination of factors that makes it to be considered one of the best hostels in Poland.