I grew up watching the original trilogy (before Lucas added all the CGI junk) over and over.
Empire Strikes Back
A New Hope
Return of the Jedi
Revenge of the Sith
Attack of the Clones
The Phantom Menace
I prefer to watch these movies as an entirety "Hatchet Style" where one watches A New Hope, Empire, THEN Attack of the Clones, Revenge of the Sith and finishing up with Return of the Jedi.
In this order the Phantom Menace is useless (nothing from it really translates from Phantom to Clones), plus in the break from Empire to Jedi we get Vader's backstory as Luke's father, how he turns to the darkside and then betrays the Emperor in Jedi to redeem himself. If you're going to do a marathon (or even introduce someone to the movies for the very first time) I would recommend viewing the movies (sans Phantom Menace) in this order: IV, V, II, III, VI.