Well the Amiga was more purpose built for Multimedia/Games in mind, Sprite Hardware and amongst other things.
If you remember the reviews of the A1000 there was somewhat confusion surrounding it when they aimed at the Business Market.
This was due to all the graphical and sound demos they showed which were not really deemed useful for business application.
The Mac on the other hand as i understand it is just a big Bitmap Graphics display. It doesn't sound much but due to the more simple and straight forward approach it makes it pretty usable at any graphical task at the time.
For example it makes it more ideal for High Resolution display and Polygon based drawing. But less ideal for 2D Platformers etc.
But the truth be told it didn't need any Sprite Hardware as it was powerful enough to do such things and you could always get a full 256 colour display or more in later models.
In the long run the Macintosh graphical route was a better option for supporting it today, In the day the Amigas option was better to get max performance.
Edit: To answer who was king in Graphics i would say the Mac was when the first colour models arrived, But the Mac didn't have anywhere near as many games. But the games that did come on the Mac like Lemmings and Worms are much superior versions, Much more colour and 640x480 resolution etc.