It's hard to say for sure, they are all so different. My experiences in classical music and opera are fairly new. However, I would have to say that my favorite composer in terms of the operatic genre would have to be Mozart (though I haven't heard many yet) I love Don Giovanni.
However, I love a lot of Beethoven's work, specifically his piano sonatas, I'm playing the one commonly known as the "Moonlight" sonata now but all of them are wonderful.
The one thing I don't like about Mozart's music is that a lot of it seems to be very similar. Very rarely new. Again, my knowledge is limited but listen to the overtures to Don Giovanni and The Marriage of Figaro and you'll see what I mean.