Definitely flys, but not machine flys, dumbbell freeweight flys.
Do incline flys, flatbench flys and decline flys. Do all three, three sets of each, 12 reps per set. Vary weight, speed of movement and number of reps (8 to 20) over sessions. Work pecs twice a week, tops. Different things work for different people, so note your results, use your judgement and try variations.