Hard to say, cause it depends on your Town Hall level and attack strategy. Air Defenses are unmatched vs. air units, while Mortars are useful vs. ground units. Against large troops you want your Wizard Tower to do the work, and focus Archer Towers and Cannons on single, high HP tanks.
Overall in our opinion, Wizard Tower is the most all-rounded defense as it has decent splash damage, good range and can target air and ground units as well. At higher levels, it's Eagle Artillery and Inferno Towers that gain the most attention from the attackers.