There was a short, plain-looking man with a pockmarked face walking the streets of Vienna in the early 19th century. No wonder all women he proposed to rejected him and so he remained single all his life. The poor thing! As a boy, much to the chagrin of his father, he showed no signs of being a child prodigy; he was often crying, as he was forced by his father to play the piano.
And yet most scholars, musicians, and music lovers worldwide would agree with this statement: "A universal genius widely regarded as the greatest composer who ever lived, Ludvig van Beethoven dominates a period of musical history as no one else before or since." (Encyclopaedia Britannica, Volume 14, p 737a)
Music scholars and composers consider these music genres as the main ones: piano sonata, piano concerto, violin concerto, opera, mass, symphony, and string quartet. Most scholars would agree with my evaluation here:
Best piano sonata ever: Beethoven's Piano Sonata No 29 in Bb, Opus 106, "Hammerklavier" (1817–18)
Best piano concerto ever: Beethoven's Piano Concerto No 5 in Eb, Opus 87, "Emperor" (1809)
Best violin concerto ever: Beethoven's Violin Concerto in D, Opus 61 (1806)
Best opera ever: Mozart's Don Giovanni. But Beethoven's Fidelio, Opus 72 (c1803–05; 1814), follows very closely.
Best mass ever: Beethoven's Missa Solemnis in D, Opus 123 (1819–23)
Best symphony ever: Beethoven's Symphony No 9 in d, Opus 125, "Choral" (1822–24), or his Symphony No 5 in c, Opus 67 (1807–08)
Best string quartet ever: String Quartet No 14 in c#, Opus 131 (1826).
The string quartet is my favorite music genre because it's the most concise and elegant of all music genres—it expresses deep thoughts and ideas in a few phrases, just as mathematics expresses the deep secrets of the universe using a few symbols. The languages of music and mathematics have the same ground of being. But even though there have been a few people fluent in both languages, most music geniuses have not been great mathematicians and vice versa; Beethoven often struggled with basic arithmetic.
Furthermore Joseph Haydn, the father of the string quartet, considered it as great conversations with nature. And many scholars consider the string quartet as the pressure cooker of music, the most demanding musical genre, the brightest jewel in the crown of music. It is by far the best medium to write absolute music.
Moreover Beethoven spent the last two and a half years of his life writing nothing but string quartets—Opuses 127, 130, 131, 132, and 135—when he was totally cut off from society because he was stone-deaf. Doesn't this fact tell us something? He was sensing that his life on this gorgeous planet was coming to an end; he loved nature deeply and took long walks to gather musical ideas, which he jotted down using a carpenter's pencil. So he zeroed in on the most beautiful medium, the string quartet, to express abstract, concise, beautiful musical ideas.
Therefore "The five late string quartets contain Beethoven's greatest music, or so at least many listeners in the 20th century came to feel." (The New Grove Dictionary of Music and Musicians, Second Edition, 2001, Volume 3, p 106a)
Thus to me Beethoven is the greatest composer ever and his String Quartet No 14 is the greatest piece of music of all time. "...Beethoven next wrote the most closely integrated of all his large compositions. From this point of view, the Quartet of C# minor op. 131 may be seen as the culmination of his significant effort as a composer ever since going to Vienna. The seven movements [c#—D—(b)—A—E—g#—c#] run continuously into one another, and for the first time in Beethoven's music there is an emphatic and unmistakable thematic connection between the first movement and the last—not a reminiscence, but a functional parallel which helps bind the whole work together. A work of the deepest subtlety and beauty...." (The New Grove, Volume 3, p 107a)