What is the best troop formation for attacking other players?

No easy answer to this. It will mainly depend on which formation makes your APC the strongest. You may have bonuses via buildings, research or hero skills that favour using all the same type or certain troop types with specific heroes.

Fighters / Vehicles / Shooters is a standard formation that is often recommended against zombies and many use it in general. Although I have heard other recommend trying something different when attacking players.

Having all vehicles in the formation will make your APC fast and cut journey times a lot. As well as increase the destructive power of the formation. Good if you want to completely destroy a base as opposed to just raid and run.

Using all fighters will actually have a higher destructive power in terms of equal research compared to vehicles. but as appgamer said depends on hero as well.

generally:

fighters > shooters > vehicles > fighters.

common formations:

A fighters B vehicles C Same Type in all squads

vehicles fighters

shooters shooters