Piano Concertos? I personally would avoid any HIP recording. While I love the muscularity and the sound of the original instruments when playing Bach, Handel, Mozart, or Haydn, the piano-forte just sounds ridiculous to me for the Emperor Concerto. My favorites:
Since you have already expressed an aversion to classical vocals (something you may eventually grow to love) I'll just mention that I would go with John Eliot Gardiner for the Missa Solemnis and the Mass in C and Klemperer for the opera Fidelio.
String quartets? I'd go with the Takacs Quartet or the Alban Berg Quartet.
Least favourite pieces? Least favourite versions of pieces?
I don't know that I can answer that easily. I already mentioned my aversion to period instruments with Beethoven... other that that I might simply advise you to use a credible music/CD guide and you probably won't go wrong. You may discover certain performances that you prefer to others, but most performances by world class orchestras and performers are quite solid.
How do you rate him compared to the other composers?
I'd place him as one of the three towering figures along side (just behind) Bach and Mozart. I would also suggest that if you are deeply passionate about Beethoven you might eventually use him as a place from which to step off into other composers. I would suggest Franz Schubert next (especially his 8th and 9th symphonies, the Death and the Maiden quartet, the Trout quintet, the Impromptus, the piano sonatas), Brahms (the 4 symphonies, the violin and cello sonatas, the clarinet works) Bruckner (symphonies 6,7,8,9), and Wagner (overtures).