I fell in love with the original one for it's sheer inventiveness and goofy characters. But Alien Force switches that up with even more inventiveness, but with mature characters now. It was a welcome change and did the series much good; it made room for new, somewhat darker ideas. Some episodes were, I felt, particularly ground breaking, such as Professor Paradox and The Vengeance of Vilgax, Part 2. Also, how could I not mention this, Alien X was amazing.
Review of Ben 10: Alien Force
