I will argue that Petey is way more important than both Waluigi and King Boo. First, Waluigi. While I know Waluigi was a huge(sarcasm) boss in Mario Dance Revolution… (rolls eyes) Petey has been a boss in 6 games, 3 of which would be considered one of Mario’s main adventures. Waluigi has only been a big boss in a dance game… Get over it.
King Boo is a boss in three less games. This is because all he is, is a Big Boo with a crown. King Boo has also been in 3 less filler games. This is probably because most sports games are better with legs. Or else it’s because he’s just a Big Boo with a crown and green eye makeup. My point is that he isn’t half as interesting as Petey, who can fly, vomit slime or make people slower, toss tornadoes, dig into the ground, summon more piranhas, heck, he can even bite. King Boo can turn invisible and float. Woo hoo.
mostly taken from wiki, I added some too
Petey Piranha
Super Mario Sunshine(boss), Super Princess Peach(boss), Mario & Luigi: Partners in Time(boss), New Super Mario Bros.(boss), Mario Golf: Toadstool Tour(filler), Mario Kart: Double Dash!!(filler), Mario Power Tennis(filler), Mario Superstar Baseball(filler), Mario Basketball (cameo), Mario Strikers Charged(filler), Mario Pinball Land(boss), and Super Mario Galaxy(boss)
Boss 6, 5 Filler
Waluigi
Mario Tennis(filler), Mario Tennis: Power Tour(filler), Mario Party 3-up(filler), Dance Dance Revolution: Mario Mix(boss), Mario Striker's Charged(filler)
1 Boss, 4 Filler
King Boo
Luigi's Mansion(boss), Super Mario Sunshine(boss), Super Princess Peach(boss), Mario Kart: Double Dash!!(filler), Mario Party 8(cameo), Super Mario Baseball(filler)
3 Boss, 2 Filler