Originally Answered: What should be watched first, the Star Wars prequels and then the originals, or vice versa?
I believe that the best way to watch the Star Wars films, and truly appreciate what they have to offer is to watch them in the order in which they were made, which is 4,5,6,1,2,3. This is the way in which they were released and the order in which fans fell in love with the franchise. Due to the almost cult following these movies have garnered the 4,5,6,1,2,3 viewing sequence kind of sells itself on how effective it was.
Since George Lucas wrote and released Episodes 1-3 after he made 4-6, he made 1-3 with the intent of them being Prequels. A prequel is generally written with the assumption that the viewer is already familiar with the subject matter, and therefore they serve more as a connect the dots, than anything else.
The surprises and plot twists that are a huge part of the movies' overall success remain intact only if you view the movies in the 4,5,6,1,2,3 sequence. Otherwise you will essentially spoil the surprise element in one of the most iconic surprise scenes in modern cinema.
Also keep in mind that Episodes 1-3 are prequels. A prequel is a story that is told AFTER an initial story, however chronologically it happens to predate the earlier tale. Therefore prequels operate off of the assumption that a viewer is already familiar with the subject matter, and they serve to tie up loose ends and create a back story for the original work, or, in this case, for Episodes 4-6.
Therefore, in my opinion, I would recommend a new viewer to watch it in the 4,5,6,1,2,3 sequence for the first time. After that it's fun to mix up the order and change up the way you watch, but only after you've already seen it once through. If you do it this way, I think you will have presented Star Wars in the way as to be best appreciated, and to yield the most entertainment.