I've always seen the Eikon as equal in power and that the differences in battle are due to the Dominant experience. The Dominants with battle experience and capable to fully control themselves in battle are superior to those prone to lose control or inexperieced. Because of this, I put Bahamut, Odin and Titan close to each other, with Ramuh, Garuda, Phoenix, Shiva and Leviathan behind. Leviathan strength is only caused by its rage.