Shining's Vector Exterminator (1991) features bitmap zoomers and super fast vectors, among some other effects that were very fresh both idea and code wise.
Sanity's Arte and World of Commodore were already mentioned, which probably are the two most technically advanced demos for the A500.
A bit of a one trick pony, but the texture mapping effects on
Spreadpoint's Cube-o-matic (1991) were so ahead of their time that they left most groups play catch for a long time. On AGA machines
Oxygene's Cuzco (1994) did a same kind of feat three years later, rotating a 1x1 pixel resolution light sourced texture mapped cube superbly smooth on a vanilla A1200.