A real 'sleeper' is the MacKerras/BBC Phil. (Manchester) M6 that got issued with a copy of the BBC Magazine about three years ago or so. Even though it's in Andante/scherzo order - which I don't care for on musical grounds - and has a 'knockout' style, third hammer stroke (which I'm also not convinced of), it's still a great M6 performance.
Among my favorites are Boulez/VPO; the Pappano on EMI; Zinman on RCA; Thomas Sanderling/St. Petersburg Phil. (now impossible to find) and Eschenbach/Philly (oddly conducted by superbly played and recorded).
The Karajan has always been rated quite highly by the English, but I'm not a fan. It's awfully fast in the first movement; has trios sections in the Scherzo that go way too slowly; has an Andante movement that's dragged-out to the point of becoming a Brucknerian adagio (which it ain't), and has somewhat anemic hammer-strokes in the finale. It does have great deep bells at their two spots in the finale. It also comes with those great Ruckert Lieder/Kindertotenlieder performances with Christa Ludwig. But then again, so does Karajan's earlier studio recording of M9 - an M9 performance that both John Kim and I like quite a bit.
The Dohnanyi/Cleveland M6 is superbly played and recorded. However, rather unfortunately, It has metallic 'pings' for hammerstrokes - something that Mahler clearly indicates NOT to do.
Among ones that are easily accessible, I think the Pappano is really good from beginning to end. It's a very powerful performance without also being slow and ponderous. It's also quite well recorded for a recent EMI (which aren't always the best, these days).
But as for Mitropolous and Barbirolli, you shoud try to hear them both. The Mitropolous M6 is quite fast throughout, but it also has a proportionally fast scherzo (which many fast performances don't). If you find that you like the Mitropolous, you might want to sample Kiril Kondrashin's M6 as well. His is also fast and exciting from beginning to end.
Barbirolli, on the other hand, is among the slower renditions available. Yet, there's a sort of relentless, hypnotic quality to it that's not at all inappropriate. However, I prefer the 'live' Barbirolli M6 that was issued on Testament and fits on to one disc. Obviously, it's a somewhat faster performance (but also without an exposition repeat in the first movement).