Bulgaria and Serbia - for me, these two countries are the best for nightlife. If you are going to travel all the way to Bulgaria, then Serbia is on your way.
If you want to experience real nightlife in Bulgaria, than dont go to turist places like sunny beach or golden sands - I was there last year and its full of children - so if you are 16 or less maybe. Also everithing closes at around 1 am. There is also a lot of crime there even though it is supposed to be a "safe" place for turists.
Try Sofia or Varna! Now there.... oh, there are no word to describe...
Be especialy carefull in Varna! There is a lot of real mafia in the streets! No not kids with guns (OK that too) but real mafia! But it's nice - it gives the experience a special touch. 
What ever you do, if you dicide to go to sunny beach or golden sands - don't go outside the "city" limits at night! Oh and there is a lot of prostitution - I'm not into that, but I heard that it's the best way to get money from tourist – the girl takes you to the beach or any other remote place, where her "friends" give you the "time of your life"... 
If you need more info...
[ Edit: Edited on 25-Feb-2009, at 00:54 by bubablast ]