My Stella.

I think its Stella, who uses her magic most effectively and basically on right time... She is mostly the first to use magic against any Enemy and the last one to give up.
2. Tecna - She is also often seen using magic. Mostly Magical Gadgets.
3. Flora, Musa, Aisha - All of these do not use magic very often.
4. Bloom - Honestly In my opinion she is the last on This Ranking... As Sometimes it seems like she does not even want to transform. (its like she is avoiding magic. lol)

She is actually skilled at magic, for she had attended Alfea longer than the other Winx.

Sometimes too late... Sometimes just right... that is just the Winx (all of them).

shining fairy.

Definitely Stella. then its Musa. then Aisha or Tecna.

My Stella uses the best magic.