>>10211740I'd probably just buy a whole set of the four bros from the same line, but if you really must do it the way of choosing one figure from different lines, I'd probably do this:
Leo - NECA 1987 Toon version.
Raph - NECAs 1990 Movie (first movie) version
Donnie - Super7 Ultimate
Mikey - SH Figuarts
In all of the lines, the four Turtles are like, 85-90% identical, with some minor parts swaps, like slightly different sculpted heads, the belts/weapon holsters, etc.
I just think the above listed, the aesthetics play to the characters: Leo looks best in the toon version, because that one gives him the larger swords than any other Leo figure. Movie Raph gives him the more gritty, realistic look. Donnie works as the Ultimate, just to get a figure that is a decent reimagining of the old school toys, and the SH Figuarts are a pretty stylized cartoony look, which probably works best for Mikey.