I don't usually post, I'm more of a lurker (is it a bad thing?), but I had to butt-in this time, and say that I'm really surprised that throughout this discussion no one mentioned either
Battle Squadron , or one of the greatest "Defender" clones for the Amiga: Datastorm
http://www.youtube.com/watch?v=MYgZomThHL0&feature=relatedHere are a few others on my list,
Crystal Hammer
The Great Giana Sisters
SuperFrog
Project X
Blood Money
Swiv