Start by deciding whether you want an audio-only monitor or one that lets you see as well as hear your baby. Some parents are reassured by hearing and seeing every whimper and movement. Others find such close surveillance to be nerve-racking. Having a monitor should make life easier, not create a constant source of worry. You might find that you don't really need a monitor at all, especially if your home is small.
It may surprise you to hear that simply being able to last through the night, unplugged, with the display off, qualifies as exceptional battery life for a baby monitor. The Infant Optics was one of the only monitors in our testing that could easily do that, and then last a while longer the next day. That alone puts it head-and-shoulders above many other monitors we tested.