I have Klemperer on EMI. It's the first version I heard and it remains the definitive version for me. It just rings true and it conveys a sense of struggle and spirituality that is by turns uplifting and touching. I have heard a few other versions (although not the ones that have been highly recommended in this thread) and they all sound superficial by comparison (despite better singers, recording, etc).
One of my favourite moments, where I think Klemperer is unique, is the end of the Gloria. Instead of trying to whip up excitement by going fast, Klemperer maintains an absolutely steady and slowish pace as the music grinds through an amazing and hair-raising series of key changes. I can still remember the thrill and astonishment of hearing this for the first time ..... each time it seemed that the tension could not go any higher Beethoven found a new gear, .... again and again. And with Klemperer's steady tempo there is a sense of monumentality and power that makes it an overwhelming experience.
Out of curiosity I listen to other performances when I can, but I do not expect to ever find anything to replace Klemp.