I propose you watch them in release order. You shouldn't watch the Prequel Trilogy until you've seen the Original Trilogy. However, it recently occurred to me how they probably work well in alternating order, though I have yet to try it. Although it does ruin mild spoilers from the Originals, it will not spoil some of the bigger things. Just whatever you do, do not watch Revenge of the Sith (Episode III) until you have seen the Originals, or at least the first two. Revenge of the Sith spoils most of the Originals' reveals.
Your main options are:
-4, 5, 6, 1, 2, 3 (strongly recommended)
-4, 5, 1, 2, 3, 6*
-1, 4, 2, 5, 3, 6* **
-4, 1, 5, 2, 6, 3
-4, 1, 5, 2, 3, 6*
*Keep in mind that 3 has a mild spoiler for 6, although it doesn't really affect the plot.
**Watching 1 first takes away from the impact of its lightsaber duel, not for narrative reasons, but because the presentation is some of, if not the best in the saga, and 4's duel is among the worst.
If you want to re-watch them after you have seen them all, I strongly recommend going in numerical order, since there's nothing left to spoil. By the way, the Prequels are underrated. Star Wars: The Clone Wars (2008-2014) is a computer-animated show that is so amazing you really should watch it if you like Star Wars. Star Wars: Rebels is its follow-up, so you should watch it afterwards. Also, watching The Clone Wars before seeing Episode III for the first time will probably give it more impact, since you'll feel invested in the characters. Also, in the case of which version of the Original Trilogy is better (theatrical version or Special Edition), they both have their pros and cons.
