It just dawned on me? Shouldn't it be determined by the Kages we've seen? Let me explain.
Okay, when it comes to actual feats and stuff we've seen these Kages:
Hashirama, Tobirama, Hiruzen, Minato, Tsunade
Third Kazekage (through Sasori's puppet), Fourth Kazekage, Gaara
2nd Mizukage, Yagura...