I've got an unreasonable affection for the Böhm Ring which perhaps relates to imprinting: it was my first Ring back in the days when Philips had cut out the LP's for $3.49 a disc--a godsend for a student. Not only are the sonics--particularly on vinyl--among the best of all Ring recordings (and to my ear superior to the Keilberth), but the singing is of a uniformly high standard. Yes, there are exceptions: Rysanek is in not particularly good vocal form in Walküre, and oldies like Mödl, Windgassen and Greindl have their rough and ready moments. Still, it's hard to get everything right in a piece as vast as the Ring, and while Böhm's conducting can be prosaic, this one makes as good a case for the work as most.
I've thus far invested in only the Siegfried from the Keilberth. (And let me assure you that after a long and diligent search, I haven't been able to find it more cheaply than at MDT.) The only problem I have with the sonics, which are otherwise superb, is that the stereo stage is annoyingly narrow. Whether this jibes with the actual Festspielhaus acoustics I do not know, but it doesn't make for particularly grand or enveloping stereo reproduction.
A disaster recently forced me to re-purchase the '56 Kna Ring, this time on Orfeo, after owning it on M&A. The sound--still mono--is very noticeably improved, with more low end and more resonance. A much greater pleasure to listen to than before, and a better buy than the Keilberth, my only real objection is the conducting of Kna, which can at times be so slow as to practically grind to a halt.
I'm still waiting for a good-sounding issue of the Solti Ring. Should it be issued (as rumored) in their new Originals series, which has produced some stunning dubs (like the Price/Karajan Tosca and the Britten War Requiem--the latter of which is a slightly earlier transfer), we might finally have a CD issue worthy of the recording.