Who has been Valentino's fiercest rival during his whole career?

Max Biaggi was indeed Valentino Rossi's most intense and famous rival throughout their careers, particularly during their time together in the 500cc/MotoGP class from 2000-2005. Their rivalry was marked by fierce on-track battles, psychological warfare, and contrasting personalities (the flamboyant Rossi vs. the intense Biaggi). While other competitors like Loris Capirossi and Carlos Checa were rivals, none matched the intensity or duration of the Rossi-Biaggi feud.