This Missa Solemnis was my introduction to Beethoven's choral music ten years ago when I was about to play in a performance of the piece, and I can honestly say that I haven't found another recording since which comes close.
The reletively small orchestra and chorus used mean that there is huge clarity throughout (for once you can hear the solo strings doubling the solo voices in the 'Et incarnatus est'), the tempi are never needlessly slow, it's an incredibly dramatic rendition (the strings and clarinets in the 'Crucifixus' actually sound like the hammering in of nails), and a few highlights for me have to be the flute solo in the 'Et incarnatus est' (again!), the prelude to the Benedictus on divisi violas and cellos (again such a clear sound that you can hear the flutes doubling the viola lines near the bottom of their register), and the wonderful quintet for soloists and solo violin that is the Benedictus itself.
Oh, just go and buy it and listen for yourself!