There has been a long dispute between the partisans of J.S. Bach and Mozart as to who is the greatest composer of all time. There is also a strong pro-Beethoven contingent, and I would gladly award him the bronze medal without a second thought, but as far as I'm concerned, Bach and Mozart are the most giant of musical giants.
There is an element of the apples/oranges dilemma here, because their musical geniuses and personalities were so different. My personal preference is for Wolfgang the boy-genius from Salzburg, partly for musical reasons, partly for purely subjective reasons, and partly (my apologies to my Protestant friends out there) because he was Catholic (even if a sometimes erratic one). That's not to say that Bach didn't compose some magnificent Sacred Music: he wrote some of the best, which I have posted on more than one occasion. As a Lutheran, however, I don't see how he could have matched Mozart's stunning tribute to the Real Presence in the Eucharist, Ave Verum Corpus: