I miss the back of the original iPhone. It's still pretty cool to see someone carrying one around, except that you know that thing crawls like a snail now.
A lot of people hated the iPhone 4 when it was leaked by Giz, but I loved it from day one -- so that's what I voted for.
Really, I think I'd have to say that the next iPhone design will be my favorite. Or maybe it won't be the next one, but once they allow the screen to take up more real estate on the front of the phone, I'll be happier.
I never did care too much for the curves on my 3GS, but I still enjoyed the design. The iPhone 4 fits my personal style better. I like cleanliness, and the 4 is more efficient and clean in my opinion. I love the glass feel, just wish it was easier to differentiate the front from the back.