system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes Beethoven, Mozart and Bach better than other classical music composers?

To my untrained ear, pretty much all classical music sounds the same to me. But these 3 are pretty much known to everyone everywhere, whether they listen to classical or not. What makes them better than others of their day? What makes them better than a modern composer like John Williams? Is there a well defined "best" out of the three?
gpt
I'll try and make this as simple as possible.

Beethoven, Mozart, and Bach are not all classical composers.

Before Bach a lot of music was very simple, a single vocal or chorus and maybe an instrument playing too. Bach was ~~an early~~ a later composer in the Baroque era, this era of music introduced much more complexity with multiple instrument parts, but it was still relatively simple music, usually featuring a single lead instrument with backing. Bach is seen as one of the best composers of this era, or at least the quintessential Baroque composer.

Mozart was ~~one of the first~~ a classical composers. The classical movement put a lot more emphasis on technical excellence in its music. Instead of the Baroque style of having a single lead instrument and backing pieces classical music would have different parts for every instrument in the orchestra, each instruments sound would weave the fabric of the music.

Beethoven wrote music in both Classical and the new Romantic style. The Romantic style took Classical and made it more dramatic. Rather than being about just technical excellence it was about the feel of the music, the ebb and flow. The composer was trying to evoke emotions in the listeners with their music. The music would alter its tempo and timbre, it would quiet down to a single instrument only to explode back into the full orchestra.

Each of these musicians is considered the best of their era, each era was considered a leap forward in the philosophy of music.
 
EDIT: A few improvements. 

EDIT 2: Clarification to answer OP's question. Each of these composers is seen as the height, or quintessential, composer of their respective eras.