Hey! I have actually been to Saint Petersburg before and will go back this April with my Mum and am looking for a nice and quiet hotel, near the city center (which is in the 3-star-range from a Western European view).
The nicest hotel in town is
W St. Petersburg hands down. Worth every penny.
The best budget stay (which is so much better than a hostel even for a Western European view) is Soul Kitchen Hostel (Moika embankment 62/2 app 9
190000 St. Petersburg, Russia)
Hope this helps. Have a nice trip.