Lots of interesting observations here!

I suppose the best-represented conductors in my collection are Karajan and Solti. That having been said, let me respond to the following...

1) Fritz Reiner stereo CDs do NOT have a "mediocre sound." (I don't think their sonics deserve to be mentioned in the same breath as Furtwängler or Toscanini, where you're guaranteed a mono recording.)

2) Mahler, of course, is fringe repertoire no more. I understand that the conventional wisdom is that Bernstein played a huge role in its advocacy... but let's just say that reasonable minds can disagree on the extent and historical importance of that advocacy.

3) Furtwängler was, for me, an acquired taste. I wouldn't recommend it in the early days of building a collection. (If you can find someone [e.g.: a library] to loan you Furtwängler, perhaps sample a little, on a cost-free basis, and make up your own mind.)

4) For core mid-Romantic repertoire, Eugene Ormandy/Philadelphia Orchestra recordings are almost uniformly grotesquely underrated [in my dilettente opinion, of course.]. Also, whenever you find them, they will almost certainly be value-priced.