Juste Belmont is the strongest because he had incredible speed and power, as demonstrated in Harmony of Dissonance.
His athleticism reflects that he was the most physically gifted Belmont.
He was also gifted in magick, so he was more versatile than the pure warrior Simon, who resembles Richard the Lionheart. Juste was more cunning and intelligent, which combined with his athletic gifts and superb use of the Vampire Killer, make him an unstoppable ninja.