It would have been hard to figure out your favorite composer if you did not tell us... other than you user name being W.A. Mozart, along with your avatar of Mozart and you signing as W.A. Mozart. Ye he he!
Um, about the question, you can't really expect people to pick ONE person to be the best composer of all time, right? I mean, that's just not possible.
There are many composers that are supreme in their own specific styles. Steve Reich is a wonderful minimalist, but you cannot say that he is a better composer because Corigliano is not as good in minimalism, right? Many believe that Bach is the best, but many also believe that Mozart, Beethoven, Monteverdi, Vivaldi, Brahms, Wagner, Ravel, Faure, Glass, Saint-Saens, Duparc, Satie, Ives, Copland, Hovhaness, Prokofiev, Bartok, Schnittke, Adams, Milhaud, Ockeghem, Dufay, Poulenc... the list can go on and on.
I, in no way can say that there is one composer who is the "best." I can list about 40 of my favorites, but not one above all the rest. There are too many characteristics that need to be compared that just are not comparable. Apples with oranges...