No, babe! The question of lack of functionality of any three turtles without the fourth can be truthful only when the one of 'em is dead. 'Cus even if someone is badly ill or just deadly tired, he is still able to help his brothers.

Well, maybe the only turtle without whom they may become too scared and confused to act properly is actually Leo. Only he has such an unshakeable temper to stand even very extreme situations, which allows him to think fast and clear in the hard moments. I mean, o'course every of the turtles is strong enough but not every one can be so coldminded to refuse their own fears and motives. I mean-- Raph is just too nervous and imoulsive and that makes him easily amused and ticked off. And the anger never leads its owner to a good thing. Mikey, in his deepest heart, is brave and agressive, he isn't a softie plushy as he seems, I mean - ppl as him can be totally different innerly, but he doesn't believe in his powers which makes him very irresolute and hesitant. And Donnie, he is able to think soberly but he isn't really a good fighter. To fight you have to be agressive 'cus the agression is the deadly weapon if you can tame it and aim it. But Donnie is rather a home kitty than a street warrior. I do not say that this is bad! Personaly I think this is a very good quality for such a sweet person. But in hard moments he may feel lack of strenght to do something and that may scare him.
Again I went too deep in my reasonings... But only Leo is trained enough to use his own wild nature as a shield for his family and as a weapon against the enemies. That's a "mental nuclear gun"! ja-ja.

Your right, but there is one thing your forgetting, they're only teenagers. They haven't matured mentally all the way to adulthood, Leo got there early and his brothers are just trying to catch up. If Leo would die, no matter what they would have to overcome their fears. Donnie has done many things in spite of his fears and overcome many difficult task (not a very tame house cat). Mikey won't let you roll over him truthfully without a fight, he can be more arrogant and it will take him much more time to mature. Raph only has his anger keeping him down, he is emotionally stretched out. He needs peace and he needs to be more open with his family, if he doesn't feel good he should at least have the sense to say it the right way. Leo is a leader who helps his brothers correct themselves, overcome fears, so by that they can lead if necessary. But this is also just a scenario, no way do I want Leo to die.

Although there isn't really a fav, I still think it would be Raph. As much as they argue all the time, they seem to connect just a bit more than the others.

Leo is definitely the best brother from turtles

Mmmm....I have to agree with kyasarin25. Leonardo is definitely the best brother. Although, Donatello and Raphael are pretty good bros too.

I'm agree with mcazevedo, Leonardo is the best brother, cause he is always worried for the rest of his brothers.

I think Leo loves all his brothers equaly. It would be hard for him to pick between the three of them. Yes, raph and Leo don't get along at times but that doesn't change that he loves him. I mean who would want to pick just one when he could have all three.

It's all a matter of what exactly you're asking. Leo loves all his brothers the same, but he depends the most on Raph, for example, the City at War saga. Leo seemed pretty hurt and questioned himself a little when Raph didn't agree with him, cause they may fight a lot but they usually agree on the important stuff.

Also, I totally agree with Xile. Remember Same As it Never Was? Granted the circumstances were a little different but without Don the team fell apart and the turtles matured mentally, well at least Mikey did. You can't really tell but I think Raph had a bit of a better grip on his anger.

Any way that's just my opinion.