1. Original Series - Nothing beats the first. Introduces an incredible universe with so much potential, I was just amazed when I first saw it back at 2005.
2. Ultimate Alien - First season was great with Aggregor and while its second season was okay with the Diagon Arc, at least it was a LOT better than Alien Force.
3. Alien Force - The Highbreed Arc was probably the most interesting arc in the entire Ben 10 series. I was so eager to find out what new episode bring to the plate. Unfortunately, the god-awful third season really brings this entry down.
4. Omniverse - Easily the worst entry in the entire series. While it's definitely a base breaker and some prefer over AF/UA, I despised it for its animation quality and the ridiculous amount of retcons to Alien Force and Ultimate Alien.