Definitely it’s the Actor,
Since you don’t want the ball to be possessed by any controller (AI/Player) You should go with Actor 
let me put all of this in an easy way:
-The Actor is the base and u can consider it as container that hold special types of Objects called Components.
-The Pawn is an Actor that is the representation of the character in the game world.
-The controller (AI/Plyaer) is the Pawn brain that controls it.
-The Character is a special type of Pawn that have a built-in player controller
-The game mode defines the game rules and conditions