Hi,
I say Rainbow Islands. Because I didn't have many controls to remember, or many directions to travel in (up or down, but preferably up) and it's cute. Plus all the power-ups, bonuses and hidden things make it the best. I also really like Bubble Bobble, but RI has the edge.
It also has longevity. I first played it around 1990, and the last time I played it was last week. And it's the game I've wasted the most hours of my life playing.
I don't expect it's the best Amiga game ever... But if this were "Amiga BEST game ever as picked by zudobug" then yes. First prize. :-D
-zudo