Tough question since much of the Amiga's library exists on other platforms (PC/ST/Genesis/Snes) and a good number of Amiga ports are less impressive than they could be. I'm a fan of the Bitmap's stuff - Gods, Chaos Engine, Speedball 2 but even they're not really representative of the Amiga (so many Sega owners think of 'Soldiers of Fortune' as a Genesis game).
If you want something 'quintessentially Amiga' then I'd have to go with Cinemaware - most of their titles were pretty interesting and the Amiga versions were always the best. That or show them DPaint
