What is the best for a digital camera 2100mah or 1200mah 1.2 v?

A 2100mAh battery is generally better for a digital camera than a 1200mAh battery, as it has a higher capacity, allowing for longer usage between charges. The 1.2V indicates the voltage, which is standard for rechargeable batteries; thus, the higher mAh rating is more beneficial for extended Photography sessions. However, compatibility with your camera model should also be considered before making a decision.