There are alot of great recommendations and comments here. I also like to see Bob Greenberg's set mentioned - I know him personally, and have always wholeheartedly recommended his series to "laymen" who are interested in learning more about classical music. For the more advanced musical students, they are a little simplistic, but his "word-score" concept is a fantastic way of explaining the basics to non-musicians. There isn't anything better out there, though I also love to recommend the famous composer Aaron Copland's book, What To Listen For In Music.
For me, the 40's/50's way of doing Beethoven is very slow and overly romanticized, though the best of them do make it work musically. Of all the older sets mentioned, I would prefer the Bohm, though he is not normally a conductor I would choose. That Beethoven set is in my opinion his very finest work. Another old set which I am very surprised at the lack of mention here, though, is the Szell/Cleveland set. While I don't always like his tempi, there is some very fine playing in that set by the Cleveland Orchestra.
Of the more modern conductors, by miles the best original instrument set is Gardiner's, which has been mentioned several times already. I am also a big fan of Claudio Abbado - those DVD's he did in 2001 with Berlin are absolutely amazing - Beethoven as it should be done on modern instruments. Incredible interpretations and performances, which always trumps sound quality IMO.
For Brahms, I also really enjoy Abbado's old set from the 70's, which was done with a different orchestra for each symphony, which is kind of interesting in itself. Also, the Szell/Cleveland set here is still considered by many musicians to be the gold standard. I wasn't going to be negative, but I cannot resist adding that I am generally not a fan of Toscanini for the German rep - the younger Italians, Abbado and Giulini do it MUCH better. I actually have never heard the Giulini Brahms set you guys have mentioned, I will have to try to find that on LP.
By the way, someone made the comment that the faster tempos were "how the early music people think they were played at the time," or something like that. This is now beyond question, with all the research that has been done in the last twenty years or so. Beethoven's metronome was indeed just fine, and he really did intend for the tempos to be that quick. And in Haydn and Mozart's time, tempi were often even quicker for Allegros, etc. Also, one of my personal pet peeves is how slow many conductors influenced by the greats of the 40's and 50's take Andantes. Andantes are supposed to be good walking tempos, as Gardiner and Abbado and others take them. And they definitely were so in the 18th century. OK, I'll get off my soapbox now.
