I was surprised to learn that the CSU has always topped the poll in Munich.
One thing I am struck by is just how well the CDU/CSU do in large, more bourgeois urban centres: the SPD were unable to win Munich from the CSU even in the Schröder years, and the CDU have won Berlin (of course by no means a uniformly bourgeois city) in the last three elections.
To an extent this mostly tells you that poll-topping isn't the most important thing: in a lot of large old cities the Greens were polling a solid share of the vote even in 1998, often close to double their national percentage. A couple of other points to consider: firstly, at all of the best elections - both absolutely and relatively vs. the CDU/CSU - for the SPD in the period covered by these maps they were led by a man whose appeal to the North German electorate was phenomenal, but who was never received quite so well further south, particularly when compared with other electorally successful SPD contenders, who always did rather well in the places in question.* Secondly, and this is really only relevant for Munich and to a lesser extent Nuremberg (it does not apply to Frankfurt or Stuttgart, say), the 1990s and early 2000s were the peak period of a certain Bavarian political particularism that expressed itself in notable CSU overperformances relative to their sister party in federal elections and some massive landslides in state polls. You see this particularly starkly, for incredibly obvious reasons, in 2002. Worth contrasting some of the numbers in 1990 with those a decade later to spot this.
*Even though they, too, were northerners. But this sort of thing is ever more a matter of tone than geography and is often beyond our full comprehension as psephologists.
