I dont know where the 'best' place is.... but I have been to a place called CAMP4 or something like that. I cant remember exaclty what the address is, but its on Karl Marx Str, across from the international Kino, not to far from Alexander Platz. There is an underground station right there too. And by right there, I mean the underground exit is 10 steps from the door....
Anyways, they seem to have a good selection, and fairly knowledgible staff.