My wife and I stayed at Castlerock in Edinburgh, Scotland - our ONLY hostel stay, so I guess it was memorable!
5 nights in a tiny private room and then, on the way back through town after a motorcoach tour, 1 night in a coed dorm room with 11 others. Great place, continental breakfast for free and great laundry service. A really nice place at the foot of Edinburgh Castle. We would do it again in a heartbeat - more fun than a hotel and MUCH cheaper.