Most of my top 5 DC characters are magic users, so this is tough for me. Spectre doesn't really "cast" magic though, he is simply a supernatural force, so while he is my favorite DC character (Corrigan version), I decided to give the vote to Kent Nelson (Dr. Fate). Zatanna got consideration as did Phantom Stranger and Madame Xanadu (neither of whom were on the list). Tim Hunter, Amethyst, Arion and Constantine would be a step behind those for me.
-M