If you wanna see the world of TMNT blow up, and everyone die, you want Mikey to be the leader. If you want it to be boring, to coplicated/scientific ( Tmnt's scifiACTION not just scifi ) and hard to understand, you want Donnie to be the leader. If you want the whole episode to revolve around violense ( Gory violense ) and very little sucess, you want Raph to be the leader. And if you ( like most people ) want the turtles and the city ( Mikey ) to stay intact, you want Leo to mantain leadership. ( Sorry if I offended anyone, thats just how I think the show would end up being like with the other turtles incharge )
Leo takes a lot of the burden obviously. But if i had to choose a backup...its Donnie. I mean Leo has a quick act, but Donnie...he comes up with the the life saving ideas that save their shells...CONSTANTLY!! and he's under a lot of pressure alll the time. How many times is someone saying "Anyday now Donnie!?" HOW AGGRAVATING would that be for you? In a sticky situation..Donnie has the destickifier already invented! haha
They all have their inputs but honestly...without Donnie the whole shebang would fall apart!
Yes, if Raph was, it would be a hot mess, as you can see, Donnie is a good leader, but I don't think he would like all of the attention snd having to plan instead of working in his lab, and Mikey's just immature and his brothers wouldn't take him seriously.
I think Leo is best suited to be the leader because it is so: Mikey is funny and sweet, Raph is strong, Donnie is smart and sweet, and Leo is funny, sweet, strong and smart. (But that's just my opinion.)