For anyone who's heading over to Bulgaria next June, I acquired some knowledge of the local tipples during my four-month exile in Blagoevgrad. Bulgarian beer should be served ice-cold, as many of them have a dark-ish taste (except Zagorka, which you shouldn't drink anyway).
Shumensko - great stuff, easily the best of the bunch.
Zagorka - The devil's urine. Avoid. Think of Dutch Gold with a dash of uranium.
Ariana - Distinctly average...
Pirinsko - Shumensko's inferior cousin. Same brewing company, I think, but the latter tops it by some margin.
Stolichno - Almost forgot about this. Bulgaria's answer to Guinness, but much lighter, easier to drink and 6%, so it gets you fairly messed up.
The most widely available foreign beers seem to be Beck's and (shudder) Tuborg.