I'm fine with the glass back but would prefer the aluminum back like the OG iPhone. I say OG iPhone back > iPhone 4 back > iPhone 3G/3GS back.
I think as of sturdiness.. it should be Original iPhone > iPhone 3G/S > iPhone 4.
But actually, i think the best iPhone would be a sweet mix of all 3. An aluminum or maybe liquid metal back, with the rounded back of an iPhone 3G/S, and the front of the iPhone 4.
That plus a 4" screen, and nice 1GB RAM and Dual Core A5 chip, + some more good specs, and that would be my ideal iPhone.