I currently own 6 versions of Mozart's Requiem, and I'll review them here.
1. My current favorite is Bohm's: Mozart: Requiem. It possesses all the virtues of Karajan or Bernstein's, with the recording quality enjoyed by Abbado's. Highly recommended. Back-story: I though five versions were enough; then a friend criticized this recording, saying, "Bohm conducts Mozart as if he were Brahms." Well, that, I thought, is exactly the how I like it! If you don't, well, then skip this one, as well as the Bernstein and the Karajan.
2. My former favorite was Bernstein's: Mozart - Requiem / McLaughlin, M. Ewing, Hauptmann, Bayerischen Rundfunks, Bernstein. (It is also availabe in a box set of Bernstein's recordings of Mozart, which I'd heartily recommend to anyone: Mozart: The late Symphonies; Great Mass in C minor; Requiem [Box Set].) I cannot say that this is perfect, but it is the way I like to hear the Requiem: slow, stately, and solemn. I don't think anyone excited about the principles of period performance is going to like it, but I believe if Mozart had the resources available to a modern orchestra, he'd want to hear something like this.
3. About 15 years ago I decided to learn about classical music - not because I actually enjoyed it, but because I wanted to be a cultural person. One of the first recordings that I genuinely loved was Karajan's Mozart: Requiem. I actually prefer the music here to Bernstein's: it is even more beautiful and moving. It hurts me to subordinate this to anything, but unfortunately this recording is older and fuzzier than Bernstein's. The individual singing parts and instrumental lines are much clearer in the Bernstein. If you have a decent modern stereo system, or fancy headphones, you would want to hear the Bernstein at least occasionally for its clarity. Also, to be fair to Bernstein, there are moments where I prefer his version. (Be careful. If you live abroad this recording is also available with a Byzantine icon on the cover. But there is at least one other recording by Karajan of Mozart's Requiem, which is not the same as this one, and I don't own. Pay attention to the names of the soloists and you'll be fine. DG seems dedicated to fooling people into purchasing multiple editions of the same recording.)
4. Abbado: Mozart - Requiem / Mattila, Mingardo, Schade, Terfel, Berlin Phil., Abbado This recording is by far the most clear. If you want to hear the distinct parts, the instrumentation, as if you were standing on the podium, this is the recording for you. It is played faster than I like, but it retains most of the emotional power. Comparing this recording to Bernstein's is very instructive: I'm sure Abbado took fewer liberties, and purists will prefer this one.
5. In Brilliant Classic's Mozart Edition: Complete Works (170 CD Box Set), it is conducted by Nicol Matt and performed by various Dutch artists. The music is not recorded as well as the Bernstein or Abbado above, and it is taken too fast. Yet I still enjoy listening to this one for contrast.
6. Gardiner: Mozart: Requiem / Bonney, von Otter, Blochwitz, W. White, Gardiner. This is a recording only a mother - or a very serious enthusiast of period performance - could love. It is played far too fast, the instrumentation is much too thin. I cannot enjoy this, and I don't think I've sat all the way through it more than twice. (Notice that the enthusiastic reviewers of this one talk about "accuracy" and "authenticity" and so on rather than "beauty" or "power." Well, that's their thing, and I'm glad they have a recording to love. But it doesn't work for me.)
I hope to consider more as my collection grows. If you have a favorite or one that you think I'd like, please let me know in the comments.