Aside from a brief spell in the late 50's/early 60's, Lorin Maazel never enjoyed much critical praise, rather unfairly in my opinion. I had the pleasure of hearing him conduct Mahler's Fifth with the Philharmonia at the Gateshead Sage a few years before he died and it was one of the greatest concert experiences of my life. Unfortunately his live Mahler cycle from that late period (on Signum Classics I think) is very indifferently recorded and is not to be preferred to his studio cycle from the 80's which I always rather liked, especially the Fourth with Kathleen Battle. There's also a live set with the NYPO available online: I've downloaded the Third and the Ninth and they're almost perversely slow.

Yes, it would've been interesting to hear Celibidache's take on Mahler given his obvious Brucknerian credentials. I'm sure I read somewhere though that he hated Mahler which, if true, I find very difficult to comprehend. But ol'Sergiu was a very strange, if great, man!