My favourite:
Caulaincourt Square Hostel – Paris – FRANCE

There are tons of Paris hostels but this one was great as it was not only really nice, clean and friendly but also situated in the north of Montmartre away from all the tourism.
It really felt like a little bit of French culture rather than just another cheap place to stay in the city.

Coming to think of it now i cant remember the names of most of the hostels i stayed in but the majority were very good.

My fav has to be one in Sofia, Was just off the main square. Not a particularly attractive hostel but the staff were brilliant and very relaxed which made the place. Just need to remember the name of it now?

Sir Toby’s, Prague. Copied & pasted from one of several of my glowing reviews here:

I’ve not stayed in loads of hostels—only in Norway, Denmark, Sweden, Germany, and Czech Republic. But hands-down the absolute best is, by far, Sir Toby’s in Prague.

Excellent excellent excellent, and I don’t use that superlative often. Clean, everything in good working condition, cool rooms and nice WC/shower rooms, great kitchen, cool cellar and patio, facility had “character” yet with modern conveniences, and a great intangible “vibe.” But the absolute best were the staff—5 star hotels should put their staff through Sir Toby’s training. Very helpful, genuine, down-to-earth, it was always great to interact with them—whether getting restaurant recco’s, tram tickets, dropping off luggage, or buying internet time, whatever—excellent.

Fabrik (Berlin)
This is not a party hostel. But it is in a fun bohemian neighborhood.
Fabrik was quiet and comfy, with friendly staff and a nice cafe. It was kinda fun during World Cup.