Will's right that 1850 is great, been there once and the amount of choice is incredible, but damn is it expensive.
If you are looking for something cheaper and considering Bulgaria I'd go to Bansko. A few places on the web rave about Borovets but having been to both there is really no comparison. Borovets is a throw back to the soviet east with hotels and food to match and the resort is poor, the smaller mountain has badly laid out runs that are poorly maintained and get swamped and iced up by beginners and the higher mountain is very dependant on the gondola being open, which is most definitely not a given.
Bansko is a much newer resort with a much wider range of slopes, newer lifts and cannons, a lot better maintained, and also a lot more choice for beginners. I stayed in a virtually brand new hotel with great facilities, flights, transfer, board hire, lift pass and lessons (if you want them) for just about £400 this year.