King Boo broke out of his painting ON HIS OWN. Yeah, he's definitely far more competent than Bowser.
Also the fact he didn't resort to anything other than his own power to fight you.
Poor Luigi always shivers at the mere thought of a ghost, yet keeps pushing forward anyway, even long before he found out his brother was kidnapped. That's how badly he wanted to be a hero and he pulled it off magnificently.
It's subtly implied that E. Gadd went to Mario before requesting Luigi's help, which would explain his being captured. Looks like E. Gadd was right about stepping out of his brother's shadow.
Holy crap, does King Boo pull out all the stops to get ol' Weege this time around. When you abduct someone mid-teleport even at the risk of losing the MacGuffin that could thwart all your plans, you're gonna be out for blood, and King Boo's restrained snarkiness showed little different. Alone, trapped in complete darkness, stranded in a pocket dimension of which the very fabric is controlled by a being who you know absolutely despises you. What does Luigi do? He mans up, swallows his fear and takes the fight to King Boo on a turf where he has absolutely no advantage to speak of. Nobody can question if this Cowardly Lion has some serious brass ones.
Also, King Boo even manages to fool and catch the player off guard, by doing things like making it look like E-4 was an end-boss stage where you fight King Boo. It's not as much Luigi, but still.
The fifth boss is one for Next Level Games. It's basically Boolossus done right. Then, for the Three Star rank, it becomes just as hard as Boolossus was.
Defeating a Boo... in a room that's extremely narrow.
The third boss, That One Boss-worthy as he is, nevertheless gets points for his clever Combat Pragmatism. Instead of possessing something specific like his brethren do, he possesses the whole arena (the Belfry Clock)!
The penultimate boss deserves mention, too. The possessor for this fight pulls off a trick none of the others had yet managed; splitting itself to form a Wolf Pack Boss with a few suits of armor. After going a few rounds that way and being put on the ropes, it then proceeds to one-up itself by flying out of the mansion... and possessing a gigantic suit of armor to try to squash Luigi.