Lets see:
Mega Metagross: Tough Claws boosts direct hits by 33%, or 1/3. Bullet Punch has a power of 40, plus STAB, 60. 60/3 is 20. 60+20 is 80. Mega Metagross' Bullet Punch has a power of 80.
Mega Scizor: Technician boosts moves under 40 power by 50%, or 1/2. Bullet Punch has a power of 40, plus Stab, 60, plus Technician, 90. Mega Scizor's Bullet Punch has a power of 90.
Mega Lucario: Adaptability boosts STAB by 2, not 1.5. 40x2 is 80. Mega Lucario's Bullet Punch has a power of 80.
In conclusion, Mega Scizor's Bullet Punch hits the hardest, at a power of 90.