GOP if we're talking about voting them into power, but to be honest in the situation Greece is currently in, where actual neo-Nazis pose a real, tangible threat to take over the government, I would be tempted to protest vote KKE (or some similar party) as a protest against Golden Dawn.
Wouldn't that make the situation worse? I always thought part of the issue with governing in Weimar was that it was so difficult to form a government with large anti-democratic parties of both left and right. I think in the last few Weimar elections Nazis + Communists was more than 50% of the vote, which made the country ungovernable.
To be honest I think Golden Dawn has reached a point of significance where it is so extremist and threatening that protesting against it is more important than ensuring short-term good governance. Obviously I'm saying this from the standpoint of someone who doesn't live in Greece -- perhaps if I did, I would think differently and vote ND. But I don't.