Machete order is pretty much the "de facto" way to watch them
Basically you watch 4, 5 to fall in love with the series and the big spoiler that 5 has. then you move to 2, and 3 to see how he got to where he was, and #6 to wrap it all up in a nice tidy bow.
Until you get to #7 lol
and #1 is pretty shit, I just rewatched 1 - 3 and found them all watchable; even 1 but its pretty bad and doesn't really do anything to add to the trilogy and just add's kind of pointless shit to it (Midi chlorians, the real age difference between Natalie Portman and Anakin, Darth Maul being a jobber)