1. Chronological Order
The Phantom Menace, Attack of the Clones, Revenge of the Sith, Star Wars, The Empire Strikes Back and Return of the Jedi
There are fans who subscribe to this order because it keeps the upbeat ending of Return of the Jedi as opposed to the more solemn, downbeat ending of Revenge of the Sith. Two big problems however is the difference in filmmaking techniques. The visual effects of the Prequel Trilogy are much more advanced than those of the Original Trilogy and will be especially noticeable if you "jump back" in time. The biggest drawback, however, is the big revelation at the end of The Empire Strikes Back. While very few people don't know that Darth Vader is Luke Skywalker's father, Empire's big reveal won't pack the emotional or dramatic punch if the viewer already knows this because of the Prequels.
2. Release Order
Star Wars, The Empire Strikes Back, Return of the Jedi, The Phantom Menace, Attack of the Clones and Revenge of the Sith
This is the viewing method with which I subscribe. It's the order each of the films was released so most people saw it in this order already and Empire's big reveal is kept for new viewers. Of course, the one drawback is finishing the Saga on a downer note with Revenge of the Sith. Still, I think this method is best and that drawback will be obsolete once the new trilogy comes out later this year.
3. Splitting the Original Trilogy with the Prequels as Flashbacks
Star Wars, The Empire Strikes Back, The Phantom Menace, Attack of the Clones, Revenge of the Sith and Return of the Jedi
This is the most convoluted viewing order that tries to counter the cons of both previously mentioned orders while keeping their pros. Empire's reveal is maintained while still ending the Saga on Jedi's upbeat note. However, narratively it interrupts the flow of the Original Trilogy by splitting it up. The momentum will be lost by jumping back to the Prequels and then returning to Jedi as the finale. Of the three methods, I consider this to be the weakest.
However, the release of the new trilogy will render the downbeat ending of Method 2 obsolete as there will be a new set of films to watch after Sith. Of course, the debate will continue because even though there will be a new trilogy, there will be spin-off films that will take place at various points between these movies. Thus, fans will try to figure out when to watch those films so that they form one long narrative.
Ah, the life of a geek...