Mr.Nutz, Brian The Lion, Lionheart and Stardust.
I still think Mr.Nutz is the most impressive of all of those, since it does have a lot of parallax, lots of colors, full speed AND a lot of stuff moving on screen at the same time. Lionheart does some amazing stuff too, but there are never more than 1 or 2 enemies on screen at time because of this.
Brian The Lion does have some amazing effects emulating the SNES Mode 7 which are quite impressive. Shame the game is kinda so-so.
Stardust is a lot of smart coding and pre-rendered stuff, I don't say its pushing the Amiga at all, but it's really breathtaking to see it in action.
I also want to note Kid Chaos for its insane parallax.