For Beethoven's piano literature, Alfred Brendel is known as the new standard.  I really like Brendel's early Beethoven, and his recordings of the piano concertos, but I really believe the ultimate Beethovenist is Emil Gilels.  It comes down to one thing: Brendel is often wimpy, but Gilels always has that great beefy Beethoven sound.  Also, for the later sonatas (which everyone MUST own) Maurizio Pollini has a wonderful recording on Deutsche Grammophon.
    A collection of Beethoven's symphonies is a staple part of any classical music collection.  Many consider his symphonies the best that have ever been written.  Recently, the 7th symphony has become my favorite of the series.  Naturally, there are many fine recordings of these works, but I would reccomend any that are conducted by Karajan.
    The violin sonatas should also be mentioned here.  Particularly the 5th and 9th, which are often available together on the same CD.  The bargain classical label NAXOS has a great recording of both of these sonatas.  And NAXOS CD's are cheaper than dirt.