Might be more elaboration than what's necessary to answer, but! This is a good question, and I couldn't resist digging into the thought process a little bit for each one 🙏
MM1 - Elec and Guts fill the role fairly equally. Whoever is "in charge" at the moment would be dependent on situation, I imagine. Guts is more bombastic, but he also tends to be more personable. Elec, while fairly level headed, also has an arrogance streak that doesn't bode well in some scenarios. Keeps the balance.
MM2 - Quick acts like he is, and Flash tries to take charge in response, so Metal ends up trying to pick up the pieces before anyone else can get into a fight over it. Truthfully, though, if anyone was to be the "official" leader, it would be Wood. He's just... subtle about it. It's more about his presence than anything else.
MM3 - Top. I can't imagine any of them arguing over who gets the title, or having much of an outright discussion about it, at all, but Top has that natural charisma. He sort of just, fell into the role after everyone else stepped back.
MM4 - Bright, though Ring and Pharaoh are kind of like his (self appointed, in Ring's case) co-leaders. They have have a bolder approach to things than Bright does, but there's not much friction outside of some annoyance on Bright's end towards Ring and Pharaoh getting caught up discussing things between each other, (unintentionally) leaving him out. Skull will sometimes make himself known in order to give them his two cents, but I think he's more of a behind the scenes guy than anything else.
MM5 - This one's a little tough. Crystal would attempt to act as a leader of sorts, but the others probably don't always take him as seriously as he'd like them to. Truthfully, I don't think they'd have one, at least not in a traditional sense. Star and maybe Gyro would also step up to the plate if they thought they needed to (for their own vastly different reasons), but it's not something that lasts once the moment has passed.
MM6 - Considering the 6ths aren't all from one creator/line like the groups from the previous games, and (I assume) were returned and didn't stay with Wily, afterwards, I can't imagine there would be anyone in a leadership role, here. To me they're all friends through unfortunate circumstances and that's that. Not a specifically structured team that would require, or come to acquire, a leadership role.
MM7 - If there was one, Shade would probably be top contender. I have nothing to really back that up like I do the others, it just seems right. However, I fear they may be a bit too disjointed of a group to have a connection that goes beyond "we have to work together now", so that leadership role doesn't mean all that much, at the end of the day.
MM9 - Similar to the 6ths, but for different reasons, I don't think there would be a leader, here. They're all on pretty equal playing ground, imo.
MM11 - Again, like the 6ths (and 9ths), due to circumstances I doubt they'd have, or want to have, one of them as a "leader."
I... admittedly don't think about the 8ths, 10ths, or the KGNs too much, lol. So, unfortunately, I have no answer for those 😔