Mengelberg is an interesting choice! I recall reading somewhere that Mengelberg thought he could just make changes to Beethoven's scores because, after all, Beethoven was deaf, not to mention dead, and who would know better what would work than the conductor himself!
My choice would be Furtwangler. Just this morning we listened to his wartime live Bruckner 9th. It is driven, ferocious, and transcendent.