Here's the list of bosses:

Cheatsy/Larry Koopa - Grass Land Boss
Big Mouth/Morton Koopa - Desert Land Boss
Kootie Pie/Wendy O. Koopa - Water Land Boss
Hop/Iggy Koopa - Giant Land Boss
Bully/Roy Koopa - Sky Land Boss
Hip/Lemmy Koopa - Ice Land Boss
Kooky/Ludwig Koopa - Pipe Land Boss
King Koopa/Bowser - Dark Land Boss

Who do you think is the hardest boss? I think King Koopa/Bowser himself is the hardest. The Koopa Kids are not as challenging to me.

Here's my list in order from the hardest to easiest boss:

King Koopa (Difficulty: 10/10)
Kootie Pie Koopa (Difficulty: 9/10)
Hip Koopa (Difficulty: 7/10)
Kooky Koopa (Difficulty: 6.5/10)
Bully Koopa (Difficulty: 6/10)
Hop Koopa (Difficulty: 3/10)
Big Mouth Koopa (Difficulty: 2/10)
Cheatsy Koopa (Difficulty: 1/10)

I havent played this game in a few years but i remember the hardest one for me ws world 3 one, that whole world was hard

I think it's Wendy O. Koopa. She's a pain! I always needed a P-Wing in order to beat her. Plus her doom ship was also kind of hard.