Worms DC AGA was a good looking game, as was Super Stardust AGA (tunnel scenes
); hard to think of one that really pushed the 1200 though.
SOTB series is an obvious candidate already mentioned, but many games milked the A500 and the 68000 to the max. I used to love those that used full overscan!
So my mention goes to Settlers! The fact it was full overscan and never slowed down with ever increasing guys/buildings on screen (unlike say populous) was a neat programming touch.