What was Golden Dawn's best area?
Depends what you mean by "best". By popular vote, it was Lakonias, where they got 10.5% of the vote -- the only district where they broke double-digits. But they were only in fourth place in Lakonias (behind the two main parties and PASOK) and failed to win any seats there.
They are now the third-largest party across much of Attica and northern Greece; their strongest performance in a district where they came in third place (also the strongest performance anywhere they won seats) was Kilkis, where they won 8.6%, third place, and a single seat. Kilkis is right on the border with Macedonia and has a very strong far-right-wing place; it was among the best results for LAOS as well.
In terms of seats, their best result was V' Athinon, where they won 3; they won no more than a single seat in any other district. But this is mainly a function of population; they were in fifth place with 5.7% of the vote (below their national average) in V' Athinon. But, in terms of seats, I
think every party's best result was V' Athinon, since it's the most-populous district. It's kind of like saying Mitt Romney's best state was California, since he won more actual votes there than anywhere else, even if he was actually quite weak there.
So the answer, depending on how you look at it, is either Lakonias or Kilkis.
EDIT: Oddly enough, Lakonias was also PASOK's best result. It was one of just eleven districts where they came third, and the only place where they broke double digits (11.0%). A very weird place. Another notable fact -- Syriza and To Potami have the best "best" district -- in both cases, Rodopis.
EDIT 2: By popular vote, ND and DIMAR have the same best district -- Chiou. However, DIMAR only placed 12th in Chiou, and there were multiple districts where they had a better performance -- that is, 11th.
There seems to be a clear gap between Teleia and Laos, with Teleia breaking the threshold in multiple districts but Laos in none at all. Teleia and better-ranking minor parties are legitimate contenders, while Laos and everything below Laos (including Dimar and Snow's beloved Antarsya) are just minor parties no one took seriously -- even though some, like Laos and Dimar, have legitimate recent histories as serious parties.