The Little Havana Party Hostel is one of the friendliest spaces I have ever been in. Composed of 128 beds and a gorgeously decorated common room/kitchen/bar, there is always someone to chat to . The spaces are ornamented with a hint of Cuban atmosphere to embrace its rum-sponsored roots.
This hostel also provide plenty to keep you occupied. They run their own club crawl (yes club, not pub) that begins with an hour of competitive drinking games. Seeing the same people at breakfast really cements some of the bonds, making it perfect for solo travellers as well as groups. My time at this hostel, whilst hazy, was some of the most fun I have had and the amiable atmosphere left me wanting so much more.
If Kraków is on your list, it's certainly worth booking a stay here. They even sell their own T-Shirts for a tenner - Krak City Bitch!