Yeah... It's kinda hard to deny it's Leo.
- Leo is the one to stab Shredder in the lung in the first issue (though you could technically say Don gets the killing blow).
- Leo is the one to kill the Rat King.
- Leo is the one who gets the most significant micro for the plot (at least in immediate terms).
- Leo is the one to kill the Clone Shredder.
- Leo is the one to make the deal with Karai.
- Leo in the Archie series is the one to lead warriors in the future.
- Leo in the 2003 show is the one to get the training arc.
- Leo in the 2007 movie is the catalyst for much the plot.
- Leo in IDW gets the brainwashing arc.
- Leo in the 2012 show gets a lot of story attention.
- Leo in Rise has a more clear arc than the others, with Raph being a leader mostly being a red herring.
- Leo in Mutant Mayhem is... Well, he's the only one of the turtles driving the plot forward... And the only one with any semblance of a personality.