Anon is wrong that Mozart said Handel was the greatest ever, but he did admire him.
It was Beethoven who correctly rated Handel as pretty great. And Bach thought highly of him as well.
"[Handel] is the only person I would wish to see before I die, and the only person I would wish to be, were I not Bach." - J. S. Bach
And when Mozart heard this:
"Truly, I would say the same myself if I were permitted to put in a word"
"Handel understands effect better than any of us -- when he chooses, he strikes like a thunderbolt... though he often saunters, in the manner of his time, this is always something there." - Mozart
Upon hearing the 'Hallelujah Chorus' from Messiah, Joseph Haydn is said to have "wept like a child" and exclaimed: "He is the master of us all."
"Handel is the greatest composer that ever lived... I would uncover my head and kneel down on his tomb." - Beethoven
Beethoven, when asked to name the greatest composer ever, he is said to have responded: "Handel, to him I bow the knee."
In 1819, Beethoven told Archduke Rudolph: "not to forget Handel's works, as they always offer the best nourishment for your ripe musical mind, and will at the same time lead to admiration for this great man."
Händel is the greatest and ablest of all composers; from him I can still learn. - Beethoven on his deathbed