We have no significant sample to reply to this question. Some will say Stamford Bridge, most will say Allianz Arena. But really, how often have we played in those stadia? We play in there once every 3-4 years. It's moronic to make any kind of conclusions out of that negligible sample. In any case, teams change every 3-4 years to a significant degree.
But, having said that, I would have to say that Bayern away is almost surely the toughest possible game for us. First of all Bayern have more quality than any other of our rivals, and two, they are very good at home.
Stamford Bridge I think is also tough for us, due to its small size and shitty pitch. But in any case, if we had Neyboi and El Pistolero (instead of Alexis/Cesc/Pedro) back in 2012, that game would have finished at least 1-6.