Do you think it’s meant to be the most powerful digimon, or is it not anymore? A years ago, I would believe that Millenniummon was one of the most powerful digimons. Today not anymore.
Although all three forms of MILLENNIUMMON are strong, but I don't think they are close to the strongest Digimon like the x-antibody Royal Knights.
Of course, since ZEEDMILLENNIUMMON was the final boss in the game of the Digimon Tamers period, I think it might be a little less than the DUKEMON CRIMSON MODE, which was introduced as the strongest Digimon at the time, or maybe even equal.
I also believe that it was outclassed by many Digimon today, but maybe in the new Adventure 2020 it will receive a power up? Let's see what will happen.