I'm with Frederik on this, all Mahler's symphonies are fine. I used to not like nos. 4 and 8 much, but I recently got the Gielen recordings of those, and can now listen to both with much pleasure.
If you think the 6th is the best, you probably don't need a recommendation for a "best" recording of that. I think that's Bernstein's, though. Ditto for no. 7, which can really seem feeble in a less than inspired performance. I like Haitink for nos. 2 and 3. Kubelik for no. 5 (the first Mahler I ever liked), and Walter and Gielen for no. 9. I haven't kept up with all the various permutations of the tenth. I think the first one I heard was Ormandy with Cooke's first performing version. The tenth's a real stunner, for sure. Some of Mahler's best music there; I'm glad people have made the effort to give us performing versions of that. (Just think how impoverished we'd be if Salieri hadn't finished up Mozart's Requiem for us, eh? And as far as I know, Mahler's tenth was much closer to being finished than the Requiem. I may not know very far, though!!)
Anyway, the only recording of the tenth I have is the BBC Music Magazine freebie with Mark Wigglesworth and the BBC National Orchestra of Wales in, I think, Cooke's second version. I don't remember if it's any good, so I'm going to put it on right now, because I can.