Never really thought about my favourites, but Maine Road is up there! My memory of grounds I've been to with city are affected by the result, but Porto remains my favourite city away game.
I've been to some obscure places for football, but I've little interest in watching amateur football for the sake of it like some do, so I wouldn't say I've been to many obscure grounds. Germany and England aside its normally only top tier sides I'd go and watch.
In Germany I would probably say Kaiserslautern, but they have fallen from grace and it wont be the same if the stadium is half empty (or less).
Valencia is maybe my favourite in Europe (so far) but it was one of the first non city games that I did, so the novelty of it may be affecting my judgement.
The Bernabeu and the Stade Velodrome in Marseille are impressive, but I really dislike the Nou Camp. Its a dump.
Boca Juniors was as good as I'd hoped, and when I die at some point in the future, I can be content in the knowledge that I ticked that off the bucket list.