I agree with Blackstar and Silverstar on how all three of these shows don't really have any common ground that I think is needed to have a favorite poll. Other than the fact that all three series are centered on a group of girls with special abilities, magical or just physical, they don't have anything in common. I also remember reading some Winx vs. W.I.T.C.H threads before and they're like apples and oranges to me. I'll just throw in my two cents anyway.
I unfortunately haven't seen all of W.I.T.C.H., but I do think that it is the best out of all three. I loved its plot, the character designs, the action that moved along with the plot and I did enjoy pretty much all of the characters. I would have seen more of it, but ABC Family didn't have Jetix anymore and I can't get Toon Disney anyway.
Winx comes at a close second for me. Yes, the 4Kids dub did alter the dialouge, but I was basically content with what I got. It had farily simple animation, but I thought that it worked with the character designs and I thought that it was a nice take on magical worlds and universes. Personally, I thought that season two was the best in terms of character development, Bloom and her relationship with Sky in particular, and providing an interesting storyline.
Totally Spies is my least favorite of the three. I never was interested in the characters. I felt like the show had too much of the sterotype of teenage girls, focused on shopping, clothes and all of their gadgets being based on what people could call girl stuff kind of lost me, despite the fact that I am a girl. Of course, I dislike the consumer image that teenage girls to begin with.