Happy Dragon in Beijing is great, I always set up my friends there as it is in the old city, close to a great subway line to anywhere/bus to Sanlitun, has bike rentals, can book train/flight tix, offers good tours at decent prices, has a really decent cafe/bar, is close to Slowboat microbrew taproom (two minute walk south), and most importantly has a very nice English speaking staff. If you go to the great wall, greatwallfresh.com is awesome, can't even begin to recommend it enough, but only the owner's son speaks English.
Pingyao, would also like to see suggestions, never been. However, I gotta imagine its a tourist deathtrap, unless you live in China and have a weekend of nothing to burn, you probably should reconsider spending precious vacation time there.
Guilin/Yangshuo, it's been a few years (2011) but I stayed at Guilin Travelling with Hostel in the center of Guilin. Nothing special but they speak English and can set up your tix to Yangshuo/Longjing terraces (bus picks you up right outside) and are right next to the central river area, which is really beautiful at night and worth a wander.
In Yangshuo we stayed at Trippers Carpe Diem, again, great English, awesome rooms, nice cafe with a terrace overlooking a rural valley and mostly importantly, outside the town. No matter where you stay in Yangshuo, do NOT stay in town, too noisy and touristy and its only a five minute bike ride away if you really want to go out for the night.
Xi'an was great for hostels, anywhere within the city walls is probably good. I stayed at Bell Tower Int'l Youth Hostel which has a really convenient location next to the central drum tower in the old city. I would recommend it as it's not that far of a walk to the train/bus station to the Terracotta warriors/Huashan (although I'm sure you can find a closer one, jsut remember anything within hte city walls should be fine). Pro tip: spend every evening eating in the Muslim Quarter, the food is amazing and wandering the backstreets is really fun.
Hong Kong has a lot more variety, from being there three times here is how it works. If you want cheap and close to the night markets (probably my favorite thing to do is wander them at night), stay in Kowloon. I would recommend any hostel in Sincere House on Argyle Street near Mong Kok station (there's gotta be at least 10 of them, just pick a decently rated one), do NOT do Chungqing Mansions on Nathan Street. If you want closer to Lan Kwai Fang (the nightclub district) or anything on HK Island, then try CheckInn Hostel in Wanchai or YesInn in Causeway Bay/ Fortress Hill. Any of these locations is good; bonus protip: check out Lantua and Lamma Islands, but not the giant buddha (tourist trap).
Don't worry too much about ratings, these are all well-touristed sites in China so hostels are better judged in tiers and by location, worrying over a rating on hostelworld, agoda, or hostelbookers isn't worth it.