Obviously you would think Donaldson but...
Happ has been among the best SP and pitching is always really valuable at the deadline.
Donaldson has not played well at all and even if he starts producing like we expect him to you have Machado, Moose and Beltre on the market competing with him.
Of the contenders Atlanta looks like a fit if they continue to play well, maybe the Angels if they drop Kinsler and move Cozart over and the Cardinals.