Let's say you have a friend or family member who's never seen any of the Star Wars movies or read any of the books. Would you show them the old movies and then the new movies? I think the old movies should be seen first, but I know some have disagreed with me... what do you think?
Show them Episodes 4, 5 and 6, then 1, 2 and 3. I believe that the story of Star Wars is far more fascinating if watched in the order they were released.
I'd say start with episodes IV-VI. Sure you'll know what happens to Anakin and Obi-Wan in episodes I-III, but if you start with I-III, you lose the bigger twists and turns in ESB and ROTJ.
Show them ROTS thru ROTJ and skip episodes I & II all together. Don't let those two tarnish your friend's opinion of Star Wars which, besides I & II, is a great series.
I don't know if skipping over TPM and ATOC is a good idea as there are actually many good moments from those two. I tend to ignore anything with Jar Jar, Anakin and Padme. But as far as establishing Obi-Wan and Palpatine there were excellent scenes. Visually and musically, there were many excellent scenes as well (like Duel of the Fates).
My advice: watch the originals first. Once you've seen them all, then I suggest watching them in order of 1-6.
I think showing them IV-VI, then I-III is the best way to do it.