Platformers. Is there any question?
At a point in the Amiga's life you could lift a doormat and a new Amiga platformer would come out. Obviously there were piles of shit but maybe if only because of the sheer volume of platform game production, soem great games have come out that pushed their console rivals into shame.
You will see the greatest technical advancements in Amiga games appear in platform games and nothing else.
It was everybody's focus!
So to me the Amiga was a platformer's machine. Just as the Genesis/Megadrive was a shmups console.
I still wouldn't say "best genre" but more like "what genre do you identify with the Amiga" or some such shit. Still, being the most popular genre developed in it, it makes for the greatest advancemenets to be done, so one could argument it's the "best" genre.