To me, the best is the most powerful, and Samantha seems pretty high up there. Sure, her mother may have had more magical power due to being older, but Samantha was queen of the witches for a while, and she seemed to be able to do all the stuff Endora had done by the end of the series.
I really can’t think of any other witch series that comes close. I love the Halliwell sisters, but, for being the most powerful witches ever, they were awfully weak. And, as the drug-like nature of Willow’s powers mean she has to be too careful to be too powerful–though, granted, I haven’t seen the entire Buffy series yet, and am just going by what I’ve read.
I don’t believe either version of Sabrina was all that powerful, and Teen Witch gave up her powers at the end, and needed a huge boost just to become popular–plus water could wash away stuff she created. And both Wicked Witches were defeated by a little girl. The witches in W.I.T.C.H. are more like superheroines than having any real cosmic powers.
Jeannie is more powerful, but she’s not a witch. And those are all the candidates I can think of. Being a comedy, Samantha’s powers didn’t need to be limited so much for the sake of drama, especially since she was disinclined to use them, and I think that’s what makes her more powerful.