This is so tough because I feel like each Kage has an attribute that makes them the strongest.
Raikage:Speed
Mizukage: Kekkei Genkai Release
Kazekage: Sand(Ultimate Defense with a good scale of Attack potency)
Tsuchikage: Can Fly/Levitate in the Air and has access to Particle Style
Hokage: Has 100Healings Jutsu(Can Tank just about anything thrown at her and has Raw Brute Strength to with it)