It depends on how you intend to use them.
The best boxing gloves for training are usually in the 14 to the 16-ounce range, as the added cushioning will protect your hands during training sessions. Heavier gloves also prepare you for competing with lighter gloves. Using heavy gloves in training makes competition gloves feel lighter by comparison come fight night.
Lighter models — in the 8 to 12-ounce range — make for the best sparring gloves and competition gloves. The lower amount of cushioning provides a more effective power transfer when striking your opponent and won’t fatigue your arms to the same degree as heavier alternatives.