I'd say it's da Bears. 4+ hours from Detroit by car and if you've every gone to the games you see plenty of opposition colors at both sites. Plus they have a long history of playing games against each other since both are long lasting organizations and inthe same division in modern times. You can say the same for GB, but I feel that, especially recently, most Detroit fans don't really Hate GB. There's kind of an awe/jealous factor of Favre.
The Lions records against both is remarkably similar -
61-83-5 vs Chicago
63-78-7 vs GB