Any of them. The main point they are all standard, so you can use convertors, cables, chargers and so on. USB has U in it for a reason. Using proprietary ports in this day and age is so backwards and user unfriendly, that it's not even funny.
I'm more of an Apple(/Jobs)-hater than most on this site, but even I have to admit the Lightning port is so much prettier and more user-friendly. You can put it in upside down or downside up -- it works both ways!
Though, I vaguely recall reading that some future iteration of USB is going to go that way, it'll be nice to get that finally.
Lack of interoperability defeats the purpose. What use is there from such port if you can't use a commonly available charger or USB cable when you forgot your Apple proprietary one at home?
