I would say:
Rise by Tristar/Redsector
Relic by Nerve Axis
Anything by TBL
for starters... There have been a few big demos released in the last few years for AGA that are quite amazing, but you pretty much need an '060 or WinUAE to get a good framerate out of them.
I've been out of touch with the Amiga scene for a while so there may be a lot more than I'm aware of
