But why was T'Challa in Black Panther the least interesting character?

T'Challa was not meant to shine as a character. Trust me Chadwick can do incredible charisma, just look at his James Brown. His character was meant to be a relaxed thinker and doer. Everything he did was through emotional intelligence. Never got overly emotional, never got angry to the point of losing himself. More importantly, a King or a Leader is supposed to allow his teammates to shine, pop and give them room and tools to succeed. He's meant to allow them the opportunity to succeed. Ryan Coogler (as a writer-director) and His other writers constructed T'Challa's laid back strength character on purpose. That is a version of what a true leader who was raised up to be a King should be.