Located in the city center, the Smart City Hostel in Edinburgh has rooms for up to 12 people (singles are available too). Private bathrooms, clean linens, and towels are available. Security lockers, luggage storage, key card access, and concierge service are all provided. WiFi is available in the lobby. The hostel has a restaurant, too. No curfew.
Nicest hostel I've stayed at in Europe, and cheapest as well! Centrally located, friendly staff, and clean rooms... felt like more of a hotel than a hostel!