I won't claim to be a score purist or music theory scholar, but after listening to Abbado, Barbiroli, Cortese, Haenchen, Karajan, Levi, Levine/LSO, Herbig, Haitink/CSO, Fischer, Gergiev, Rattle, Segerstam, Tennstedt, Thomas, Wit, Zander, etc., I've settled on my preferences as listed (with versions by Jansons(LSO), Farberman, and Bertini in the 'honorable mention' category). My first recording of this symphony was the Abbado/CSO on LP. Over time I found that a 'bright' performance simply didn't work for me (though their M5 is still fine). The more recent Abbado/BPO or Abbado/LFO are held up as a models by some, but they, too, simply were not dark and angry enough. Instead, I think a deep, rich sound really suits the M6, hence the appeal of the Bernstein and Boulez recommendations. (The MSO recording satisfies in this regard, too.) The bells need to be really 'organic': if you've listened to a real herd of livestock with bells, you know what to listen for in the pastoral interludes: more clunk than ping and tinkle. Also, I'm a A/S and 3 hammers heretic/hypocrite: as quickly as I'll assert his own performances as justification for the A/S order, I'll cite my personal preference for the three-strikes-and-you're-out treatment for the hammer blows (especially since the last one comes unexpectedly and devilishly late upon the already-staggering victim). It's no small satisfaction that Mackerras, Bernstein(VPO), Schwarz, Levine(BSO), and Delfs all saw fit to include three hammer blows in their latter renditions.
