My favorite of the Koopalings had been Ludwig von Koopa because of his fabulous hair, though once I noticed that he only has one tooth it kind of dragged down my appreciation for him. Reason being, it's kind of distracting once you notice it. It doesn't really make sense either if he is supposed to be the oldest among them. I guess he technically still is my favorite among them since I don't really play Mario games that much and don't have a ton of firsthand experience with them. I guess Roy would probably be second because his shell colors (purple with pink accents surrounding the base of the spikes) are pretty nice and he has those sunglasses to set him apart.
It's definitely between Iggy and Roy for me. I normally don't like designs like Roy's but I love his color scheme. Iggy has cool classes and his design nails the zany genius look. At least, I think he's the genius. I'll go with Roy, I think his design is just the standout.
Roy and Lemmy. I wasn't a fan of Wendy O. until Color Splash. The boss fight was so cool and unique but most importantly, the background music got stuck, playing in my head for months/years.