As-long as you invest in them, Any character can be the strongest character.
I.E, Let’s say you just got Diluc after ascending Amber to lv 90. (Totallydidn’thappentome-)
Would you or would you not switch Amber out just to use Diluc?
I’m gonna be hopeful and assume you picked the logical answer, To keep Amber.
While five-star characters are (generally speaking) better on their own, It all just comes down to good artifacts and character levels. Any character can kick a*s as-long as you build them correctly.