Peppers come in all shapes, sizes, and heat levels.  I most often use red bell peppers when I need something mild, and serranos when I need a little heat.
Generally speaking, the largest peppers, like bell peppers and poblanos, are the mildest. The smallest peppers, like habeneros and ghost peppers are the hottest.  Also, pointy ends can be an indicator of heat.   Don’t let the color fool you. Color has no relationship to heat. Red and orange bell peppers are very mild and sweet, wonderful eaten raw. Take a bite of a green chili or serrano pepper, and you may be sorry!
Speaking of heat, don’t try to chase the heat of a pepper out of your mouth with water or beer. That just spreads the burn around and intensifies it. Instead, go for dairy. Milk and sour cream do a good job of numbing the fire. When ever you work with hot peppers, put a latex glove or at least a sandwich bag on the hand that holds the peppers. Cutting into peppers with bare hands can cause a nasty chemical burn. After making a batch of pepper jelly, I learned the hard way! To take the sting out of the chemical burn hot peppers WILL cause, follow this remedy: Douse your hands with rubbing alcohol really well. Let the sit a few seconds, then wash your hands using a hefty dose of dish soap. Repeat twice more.
Look for peppers with smooth, firm, unblemished skin. In particular, inspect the stem. Darkened stems close to the flesh of the pepper are an indication that the pepper will soon be going bad. The greener the stem the better.