Frey, clayman, and beast king are at the bottom of the demon lord food chain as they are unawakened, then there's a middle portion where the majority of the awakened demon lords fall into (stronger than current diablo, weaker than rimuru), then there's Milim, Guy, and Rimuru (dragon, demon, slime), who can beat a true dragon. Our good buddy rimuru is currently the weakest of the 3 but has the most room for growth, milim has the most pure firepower/energy, and Guy is the best overall. Veldora is also currently at the bottom of the true dragon trio, with the strongest working for Guy and the middle sis working for Guy's rival. The reason they don't rule is because Veldy-boy was a ball of chaos, and the two sisters were following the two sides of a massive war. The only reason Ice dragon was able to fight True Hero and Guy at the same time was because of the circumstances (Guy didn't want to hurt her, the hero couldn't cooperate with guy and had to attack him, ice just got a new ultimate skill in the same series as beelzebub, etc.) not because she was stronger than the both of them