1.-OS (by your animation and design of characters perfectly developed).
2.-AF (their new aliens and them 2 story arcs of the series are good and entertaining).
3.-Omniverse (their new villains as Malware, Khyber, Dr. Psychobos, Lord Transyl, Evil Bens and Maltruant were great, addition to the variety of species aliens was more large and wide)
Not tale to Ultimate Alien since its first season is me made confusing to know who was really the true main antagonist, as well as introduce pathetic characters such as Quince, Morgg, Kwarrel, Coronach and Hulka which truth shows us the lack of imagination by the creators.
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.
I say the Original Series is the best. One minor detail I hated as how Ditto and Upchuck appeard like four times while Frankenstrike, Snare-oh and BlitzWolfer only appeared like once for almost ten years. I don't care how many comics they were in, they lacked proper appearance. Even Eye Guy appeared twice technically.