I had a minor epiphany recently. I realized why Leonardo is my favourite Ninja Turtle and it has nothing to do with the characterization (that came later).
It's because he's the most balanced character to play in the original NES TMNT game. I realized this as I've been replaying the old NES game.
Leo has good reach with the katana and attacks relatively fast.
The other three are trickier to play by comparison.
Donnie is best for reach, but slow.
Mikey and Raph are near useless for reach, which is important in a game with a dangerous hitbox.
But Leo is the most balanced of them all. I played him the most in the game back in the day.
This love for playing Leo on the first NES game shifted to making him my main pick for the later arcade games. He wasn't the most popular Turtle, so most didn't mind if I picked him.
So yeah, even though I love the character now, it's because of the NES TMNT game that he was my favourite Turtle.