I assume by "at their best" this should not be about alcohol.
Leo, tea definitely. Maybe not an interesting answer since it's kind of a given, but he could certainly have a wide variety that he enjoys, both caffeinated and de-. (I'm sure the choice would vary depending on what he's up to. Bed time or before/after a workout in the dojo would surely be different choices.)
Donnie, maybe plain old water a lot of the time as he probably knows water is to the body as oil is to his machines.
Except when it's coffee when pulling late nighter, or in the morning
after said night.
Mikey... the soda and energy drink kid. Maybe BooKoo energy drink if you want a name brand.
Raph... it might vary by situation? Water when working out, a shot of 5-Hour Energy pre-battle?