I personally think that Lewa is the coolest because of his awesome mask (No Meso, its not a dumb mask), his super cool axe, and the fact this his fingers are super bulky.
Also in terms of the mask game his weapon works the best while his mask is one of the hardest to get off (with Gali’s being the hardest).
I always pretended they were machine guns, which made him the coolest to me as well
Also for being the one I have the most emotional attachment to as a set
I also sometimes pretend their machine guns lol
I like Pohatu the most, for a very simple reason: he kick
Tahu because he just naturally looks cool and was the original Toa Of Fire/original heavily marketed Toa and was very innovative (along with all the Toa Mata for that matter).
My favorite is also Lewa, for several reasons. First off, I love green as a color, and I’ve always been a big fan of air based powers. Two, I do love his mask, both for the power it gives and the way it looks. Lastly, I loved his characterization in the story.
I got into BIONICLE far too late to care about the Mata so I have no dog in this discussion, just going to make the requisite “Kopaka is the coolest because he has ice powers” joke and move on
To my little 8 year old mind, they looked like electric plugs so we decided they could zap enemies with electricity (we invented the Toa Inika 5 years before lego did lol) 
I always thought Onua looked the coolest, with steam venting out of his mask, his huge claws pushing aside the earth in his promotional image. Quite epic.
Kopaka is objectively the coolest Toa Mata, it’s literally his elemental power. Why is this a debate?
Good to know that i’m not the only one repping Onua. best design, best character, fight me
Onua because number 1. He’s the only one I have 2. He looks hella cool
The coolest Toa Mata is Tahu, because I locked him in my freezer
thank you for that, now I have a friend to play with in here
I have to go with Pohatu for this question. Brown is a color we don’t see a whole lot in constraction, especially since basically every later stone-theme character had a different color than the one before. Additionally, though some say it looks silly, the upside-down torso is really cool and sets him apart from the rest of the Toa.
