The very first iPhone is by far my favorite. Hands down. I remember getting it right when they hit the stores. Remember, nothing in the world for mobile phones was like this.
My girl and I each got one and it was blow away. We immediately tried playing music through the headphones and then I called her. The music faded for both of us, we spoke for ten seconds, hung up, and the music faded back in.
The fit and finish of the first iPhone with its aluminum/metal back and sexy looking operating system and software was and still is second to none. The way it was all packaged I think is yet to be rivalled by any other iPhone or iOS release, including the latest.
I love my iPhone 6 Plus but iOS is full of tons of bugs, it's slow and choppy at the best of times, and Apps are all blown to crap on it. It's nowhere near the "experience" as the first iPhone.
On design alone I think that first iPhone stands the test of time. Of course if it were thinner it would be better but still, it's damn fine.