I thought they all sucked. One had nothing going on, and was really boring until the end when a few people nobody cares about die. Two has the horrible romance between Anakin and Padme, and then still didn't really advance the story. I'm pretty sure a three year old could tell you Count Dooku is evil, and there are more Sith. The Third wasen't exactly a horrible movie in my opinion, it was just a horrible Star Wars movie. I mean sure they had some really nice action scenes, and some of the light saber fights were epic. But they lost all sense of that wonderment that you feel when watching the original 3. I felt like Anakin was whiny, but I liked the reason he turned to the dark side. It made him seem a lot less evil, and more of just somebody who was afraid for the one he loved. Although I thought that the weird scream at the end was a bit off, even if it did last for a few seconds.
If I had to pick one of them, it would be the first. Just for the reason that at least the other two had some pretty cool fight scenes for some entertainment, and the first one spawned the other two. The entire series could have been fixed if Anakin had started out a teenager, they had spent two movies on creatng a romance between Anakin and Padme, and spent more time on Obi-wan and Anakin.