Watch 4, 5, and 6 first. Those came out first. The prequels 1,2, and 3 came out about 3 decades later. They explain the reasons things happened in the earlier films. If you watch the prequels first, you'd be spoiled on the plot twists and such from the first films. Then you watch episode 7. Basically watch in chronological order of release.
Episodes 1,2, and 3 are considered bad simply because they weren't well written or executed, but the action and graphics are better plus it fills in essential plot points from the first three films. Definitely watch them all.
4,5,6,1,2,3, and 7