For this I'm going to have to show my brass player roots and say that,
generally speaking, Vienna Philharmonic is at the top of my list.
In terms of sound and musical ability they are at the top of my list.
However, they primarily specialize in the Germanic repitoire.  Other
orchestras I would list as among the top groups, for a mixture of technical
ability, musicality, and sound, are: Atlanta Symphony, London Symphony,
San Francisco Symphony, Los Angeles Philharmonic, Berlin Philharmonic,
Concertgebouw Orchestra, and Camerata Salzburg (primarily for their early
stuff).

-Nathan