Bulgaria offers a huge variety of conditions in its relatively small area. If you are looking for smoothly prepared pistes and latest mountaint facilities, Bansko is your choice. Here is a resume from its official website:

"The lifts and pistes operator in Bansko takes care of your maximum comfort with our 8-man Gondola cable car by Doppelmayer, five 4-seat lifts of the latest Doppelmayer models, one 3-seat lift by Poma, 6 drags and 10 baby drags. The easy access to all lifts is secured by chip-cards from SkiData.
The existence of excellent quality crisp white snow on 65 kilometers of marked ski runs and ski roads is made possible by their regular maintenance by 12 Kassbohrer snow grooming machines. Snow is guaranteed from December until May by 40 TechnoAlpin snow cannons."

"Our ski storage and rental facilities, furnished by brand new ski-rafts from Wintersteiger, offer the latest ski and snowboard equipment from major Western brands."

Source: http://www.banskoski.com/suorugenia.xhtml.en

On the other hand if you are budget concerned, look for less luxury but rather adventures you could go for many other places like Vitosha, Semkovo, Chepelare and others. My personal and best choice is Sofia's nearest mountain - Vitosha. Its ski lifts are more than 20 years old and ski pists are not maintained in the biggest part - perfectly suited for freeride snowboarding. One day on Vitosha fits in 25 euros / $35 taking into consideration I sleep at home in Sofia and spend 2 hours in transportation. This is the only way I could go every week snowboarding :)