The most important thing to look for in all kinds of baby monitors is audio quality. Regardless of whether you want a video-based baby monitor or not, you need clear audio so you can hear your baby properly. You'll also want one with sound activation so that you don't have to listen to white noise 90% of the time. With sound activation, you'll only hear the noises from your baby's room when there's something important to hear.
If all you want is a no-fuss audio monitor, the Bump and Baldwin both recommend VTech’s DM221 audio monitor, which consistently garners a lot of accolades. Using digital audio technology, the DM221 offers clear audio transmission and eliminates the crackle of analog models. A two-way intercom allows you to talk to your baby, while a five-level sound indicator can visually alert you to cries from the other room. The transmitter also features a soft night-light for your child. And don’t sleep on its compact size, which makes it perfect for travel.
Notifications and alerts work by sending a message or email to your device when motion or sound has occurred. This feature is only found in the Wi-Fi monitors, and isn't the best feature for baby because it comes after the fact (sometimes up to 30 min or more after), it does not offer details of the type of sound or motion detected, and could get annoying with useless and excessive messages being sent. In the end, we prefer sound activation over notifications and feel that alerts and notifications aren't all that useful for keeping tabs on your baby.
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 before needing a charge. (The manufacturer claims it lasts 10 hours with the display off—we got that amount of time off a full charge, even when checking the display intermittently.) This model also charges via a USB connection, which actually sets it apart from a variety of competitors, some of which use ineffective and inconvenient proprietary DC chargers or even disposable batteries.