I've been listening to different recordings of Parsifal recently and I have read through the entirety of this thread. My opinions have changed somewhat since my comments here.
Those who have pointed out that Wagner wrote the opera for Bayreuth with the particular acoustics of that theatre in mind know what they are talking about. I have been fortunate enough to see Parsifal at Bayreuth and it's a totally different experience there. I have only seen the opera four times despite having had the chance to see it more often, but I'm only interested in seeing it with a conductor and/or a Kundry who can do it justice or seeing it in Bayreuth. My first Parsifal was at the ROH because I had never seen it and it had a good cast. I've seen it twice with Waltraud Meier, which I traveled to Germany to see. The second time was with Barenboim conducting and that was one of the greatest performances of anything I have ever seen aywhere. The last time I saw the opera was at Bayreuth and you could say that Parsifal is an opera written for the instrument that is the Bayreuth Festspielhaus. Therefore, at least one recording from Bayreuth is essential IMO.
I was surprised to find that from my recent listening the best all round recording is the Kna 62 with Irene Dalis one of the best Kundrys. No, I'd go so far to say that most Kundrys are unsatisfactory and that the only singers who could produce the many facets of that character vocally are Ludwig, Modl, Meier, Callas, Dalis and Ericson. Kna understood the work like no other. Some may prefer his earlier, slower performances (52 is to be preferred over 51 and is possibly Modl's best Kundry), but I can't be without Vickers' Parsifal and so the 64 is also necessary for me.
One of the interesting things about the comments here is that the title role is not the one that people in general buy recordings for. Gurnemanz stands out as needing the right interpreter and I would argue that there's a case for renaming the opera Kundry. However, it is possibly the conductor that is the most important element here. Of the live recordings without Kna, the only one that I feel is really worth something is the Gui due to the pairing of Callas and Christoff who are both superb in their respective parts. Yes, it's cut to shreds and would never be a first (or sole) choice, but the cast is really rather good (in addition to Callas and Christoff, Panerai isn't chopped liver as Amfortas).
I've always been a fan of the Barenboim among the studio recordings, but as someone has pointed out above, whilst act two is wonderful, the outer acts aren't up to the same level and Holle is a rather plain Gurnemanz. My favourite Gurnemanz's are Frick, Christoff and Hotter with Moll somewhere behind them (yes, his is the most beautifully sung, but he doesn't have the gravitas of those previous basses).
I had always discounted the Solti as I'm not a fan of his Wagner as much as I'm not keen on Kollo, but he has such a superb cast and he's not as bombastic as he could be. His is a dramatic pacing of the work that is very different from Kna's, but that works on its own terms. Ludwig is possibly the best Kundry of all and has the perfect mix of dark dramatic mezzo and bright steel that puts her in the same camp as Callas, Modl and Meier, whilst not having technical limitations. Frick is possibly my favourite Gurnemanz, he has it all.
The only other studio recording I consider worth having is the Barenboim (Meier and Jerusalem are really superb as Kundry and Parsifal and Van Dam is my preferred Amfortas.)
There are two that are often lauded, but they are let down by weak casting and disappointing conducting. Like most people I can't get on with Vejzovic's Kundry. Her conception of the character changes with each line she delivers. Kundry is complex and we should get a range of different Kundrys - the sultry, the tigress, the long suffering, but each of these personas needs to come along at the right time. With Vejzovic one line she is one and then in the next she's a completely different woman. I get the feeling the role is beyond her vocally and she is singing for survival here. That's surprising as I very much like her Ortrud which someone has posted above. Moll sings beautifully and I understand why some consider him the best Gurnemanz (and therefore rate the Karajan or the Kubelik so highly), but I would like more substance to my head of the Grail knights. The main drawback to this set, though, is Karajan's self reverential conducting. It's not just that it's slow, but that he doesn't use that extra time to produce anything meaningful. Kna and Barenboim take time to let the music organically grow out of its own material. Karajan is just revelling in the sound of his orchestra.
Kubelik is a different beast, you can tell he really cares about this music and he knows his craft. However, I find his version is neither one thing nor another. His cast is weak. Minton is too light a mezzo to really get down and dirty as Kundry and Weikl is totally forgettable. Moll is fine, but lacks oomph, as does this version overall.
Overall recommendation:
Kna (Thomas, Hotter, Dalis) Bayreuth 1962
Mono Live:
Gui (Callas, Christoff, Panerai) RAI 1950
Kna (Windgassen, Weber, Modl) Bayreuth 1952
Studio Stereo:
Solti (Kollo, Frick, Ludwig) VPO
Barenboim (Jerusalem, Holle, Meier) BPO
N.