Best in terms of chops, Mangini. He is readily acknowledged to be in the upper echelon of drummers in terms of skill, along with Virgil DOnati and Marco Minnemann.
Best in terms of composing their instrumental parts, Petrucci.
Best in terms of tastefulness, John Myung. Out of all the members, he is the one who really knows when to hold back, when to step up, when to be soft, and when to be aggressive.