I think the Eroica is the greatest musical work ever created. Beethoven's first two symphony are rather boring, in my opinion; I am sure most music lovers will agree that the first two Beethoven symphonies are less interesting than the last symphonies of Mozart (particularly 40 and 41). What a shock to the music world Beethoven's third must have been. I recall that many listeners at the Eroica's introduction found the work too long and too weighty--it's twice the average length of classical symphonies by Haydn and Mozart. Certainly it has an intensity that Mozart never matched, although perhaps Bach did in works like his Mass in B Minor.
The Eroica was the first of a string of astonishing symphonies by Beethoven, including the extremely famous fifth and ninth symphonies (the "great" and the "choral"). But I don't think any were as consistently gorgeous as the third. In a letter about the third symphony, Beethoven wrote that it was his best work thus far and that "I think heaven and earth must tremble before us when it is performed." This was Beethoven's favorite symphony until his ninth. While the ninth may be "better," the third definitely had a more drastic impact on the evolution of the symphony, making it the most important and dramatic musical form in the 19th century, and paving the way for great symphonic writers such as Brahms, Tchaikovsky, and Mahler.