What's the latest os on iPhone 1?

Thinkig of buying iPhone 1 and was just wondering what latest os it could run on (16gb iPhone 1).

IOS 3.1.3 is the latest the original iPhone could run. This unfortunately means that a lot of the apps available on the App Store won't really work.

That hardware is too outdated.

Ios 3

You should buy the iPhone 3GS at minimum. Just saying.

The original iPhone runs up to 3.1.3. However most apps require iOS 4.3 to run.
The iPhone 3G runs up to 4.2.1.