Re: Ludwig Von Beethoven
Beethoven may or may not be the greatest of all time, but I'd be willing to bet he'd be at the very top of more lists than any other classical composer.
You're right about his symphonies, LG. They are a very easy way to get exposure to his work, while at the same time experiencing some of his best and most important work. His early symphonies are extremely accessible (though it seems ridiculous to think of any of them as being inaccessible), and it's fascinating to listen to his musical growth and development as he starts with very classical forms in the earliest ones and progresses to more romantic and monumental themes. The early symphonies are not unlike late Mozart, but with the easily recognizable glimpses of what would come later.
His violin concerto and his 5 piano concertos are also great listening, even for those new to classical music in general, or to Beethoven in particular (the same simple-to-complex compositional growth patterns heard in his symphonies is also evident in the piano concertos).
You can also get his essence in any number of smaller pieces - any of his various overtures like Egmont, or the various Leonore overtures, or his "Creatures of Prometheus."
And like his symphonies and concerti, his piano sonatas are all practically basic repertoire, and new listeners will probably be surprised to realize how many of them they recognize, having already heard them throughout their lives. Not quite as well-known are his sonatas for violin and piano, but they're great too. I especially like the "Kreutzer."
He wrote one opera, Fidelio, though I admit I have not heard it. Opera is not typically my thing. (I appreciated a scene from the old "Family Ties" TV series, where Alex (Michael J. Fox's character) has come home from the library with a box full of books, records and art prints to review so he can impress his new "arty" girlfriend. As the family pulls out item after item from the box and read their titles, they find one that's called "Opera: Why It's More than Just Annoying")
I also admit I have not spent much, if any, time with his quartets, and I view that as a personal failing of mine. They're known to be among the finest pieces of art that western civilization has ever produced, especially true for his late quartets. I've stayed away from them only because they're musically complex and intellectually demanding pieces, and in my limited experience with them it takes a lot of effort to even get comfortable with them, let alone begin to understand them. I would guess that how I feel about them may be similar to how listeners of his time reacted to his other works that we consider mainstream now. His music just seems so "right" (Leonard Bernstein famously said that Beethoven was the one composer he knew who chose the perfect note every time, that anything different would have diminished the work in some way - a concept that a musicologist described to me as "inevitability" in music), that it's easy to forget that in his day his music was revolutionary.