Just speaking for myself, I feel that Bruckner's most 'beautiful' adagio is the one to his 6th symphony. In fact, if it weren't for the finale, I feel that the 6th would be among Bruckner's very greatest works. In other words, the first three movements are terrific.
I very much like the first two movements to Bruckner's 7th, both of which being rather 'slow' in tempo and feel. In fact, the 7th is my favorite of Bruckner's 'completed' symphonies.
Needless to say, the last movement to Bruckner's 9th is a truly monumental movement (discounting the almost absurd, conjectural finale).
When it comes to Bruckner's monumental 8th symphony, it's actually the first movement that I like best. In fact, I think that I may prefer the first version of B8 to the much better known revision.
Just my zwei groschen.