Boroque: J. S. Bach, Domenico Scarlatti. By far.
Classical: Joseph Haydn >Wolfgang A. Mozart, then the KING of the classical period who brought new and exciting unknown techniques and harmonies to the era; Ludwig van Beethoven.
Romantic, in no particular order: Nikolai Medtner, Sergei Rachmaninoff, Niccolai Paganini, Franz Liszt, Frederic Chopin, and Johannes Brahms.
There is absolutely no way one can choose a single composer to be the greatest of all time, due to the different styles all of them used.
Anyone familiar with Leopold Godowsky?