In Borovets mine has to be Yasrebetz 2 (Popangelov? on the piste map?) The speed you can pick up from the top of the chair lift down to the first half is immense. Followed by the really steep bit you can flat line on a baord to get as much speed to carry you as far as poss along the long flat at the bottom.
Marky 1 also has this awesome bomb hole that can launch you massively high if you keep to the right of the bomb hole
Borovets Id have to agree with Dbut and Garrick. Love yastrabets 2. Mussala path in the right conditions is a nice run back to the resort from the top.
some fun little kickers on the right hand side of yastrabets 2 behind the cannons to get a bit of air and generally end up flat on my face or arse, have to take it slowly down or you won't even see them.