Best sounding Saint-Saens Sym No. 3?

There are many who like the Fremeaux/City of Birmingham Orchestra release on EMI (re-released on vinyl and CD, I believe, by Cisco). Very lush acoustic. Another well-recorded version is with the Dallas Symphony and Jean Guillou on Dorian, with a good pairing of the Jongen Symphonie Concertante (which is the main reason to buy that recording, in my view). The old Telarc recording with Michael Murray/Ormandy conducting is sonically spectacular (the vinyl was famous for causing cartridges of the time to mistrack), but I really think the performance in the second half of the piece is so slow and unexciting as to be unlistenable. My favorite overall is with Guillou and the SFO (DeWaart conducting) on Phillips, not so much for the quality of the sonics overall (they're OK, though a little "thick" sounding, not really as transparent as others) as for the fact that Guillou used the 32 foot stops more than most organists do in this performance--far more than he did on the Dorian recording, in fact--so you get full 16Hz notes in the second movement (or the second half of the first movement, depending on how you look at it). This one apparently has become available again on CD, worth looking for, and far better than the DeWaart recording with Chorzempa that came out on PentaTone SACD, IMHO.

Mercury has also re-issued their 1957 "Living Presence" recording with Marcel Dupre on the organ and Paul Paray conducting the Detroit Symphony. The recording has very clean sound, and Dupre is one of the masters of the French organ literature. The catalog number is 432719-2.

I don't know if I can answer your question in terms of sonic but I have gone through a number of versions of this and the ones I go back to consistently are
1)with Eugene Ormandy and the Philadelphia Orchestra with E. Power Biggs CD 2) Georges Pretre with the Paris Conservatoire Orchestra, and Maurice Durufle LP and finally one I keep being undecided over 3) Charles Dutoit and the Orchestre Symphonique de Montreal with Peter Hurford CD.
This is not counting of course the Much/BSO which you already mentioned. Hope this helps