Solek...seriously. I love how he looks up to Kopaka. He's that adorable little fanboy who is annoying, but you can't bring yourself to hate him. Or at least that's how I interpret his character.
I'm not going to speak set-wise because Av-Matoran build.
Matoro is another favorite of mine given how he is different from other Ko-matoran.
And lastly Takua because Takanuva is one of my favorite characters.