I dont think theres a "best" caliber for deer. But as close as a person can get would be a .270, 243 or a 7 mm rem. mag. but the only important part about hunting is the bullets placement and how effectivly it transfersits energy to the animal. as long as it develops about 700 ft.lb. and the bullet is placed right and does its job your deer wont ran very far, if at all.