I believe that, by the book, it's the thrower's strength. 2E had it be the strength of the thrower, with up to two points higher if the toughness of the object exceeds the strength bonus.
We had a long discussion of this on another thread
, but basically the system isn't really granular enough to give you a good answer such that throwing a baseball, a piece of Styrofoam the size of a baseball, a bus, or a piece of Styrofoam the size of a bus would give you different answers. Ultimately, a lot of it comes down to what the GM feels is appropriate for the thrower having spent a Move action to pick up the object. That might be extra damage, a chance at an area attack, or linked side effects (for example, throwing a cement truck might get you a Snare on top of the damage).