Red Superfly
You creepy little stalker
They were designed to be watched 456123, so lets just leave it at that.
They can work 123456, but are less effective.
The whole point is to watch the main bulk of the story (OT) then go back and watch the backstory once you find out Vader's secret.
You need the OT to make you care about watching Vader's origin.
Watching Episode I first is a mistake. You don't know any of the characters, they aren't introduced as well as those in Episode IV and it isn't a good movie unless you KNOW where the characters such as Anakin are heading.
Episode IV is the starting point. It's the intro to the rules of the universe. The prequels are designed with an expectation that the audience knows what has gone before.