Which is the best platform to build a game from to publish on all devices that will still look decent on each one?
I'm thinking the iphone (smallest device) then stretching it to fit the other devices.
That is not an easy question to answer it depends on your game. The best way is to do a test project place some actors around the scene and try the different options in Preview and observe the effects.
Search for "Universal Binaries" and read up on the subject. Don't be afraid to post your test project and ask peoples opinions. Most people develop in iPad and design it with a UB in mind. I jumped straight in on my first project and ended up with two binaries. Don't make my mistake spend some time now and get it right from the start. It will save you a lot of hurt later.
Universal Binary Template - Universal Binary Template Instructions Rev 4 (Short) - Custom Score Display Template