Exactly, OnEscapee is my choice too as far as AGA games are concerned. But I have the feeling we never actually witnessed a game that pushed the hardware to its limits...
As for ECS.. Shadow of the Beast 1 looked unreal for a game that ran on 68000 with 512KB of RAM! Lionheart was not fast enough, it looked like AGA but did it really push it? Elfmania is a good guess as well, with its huge sprites and good speed.