Ones that work!
A good baby monitor
is one that works at the appropriate range for you.
Try it out. Does it work when baby is asleep in the bedroom and you are working in the kitchen? A monitor is not necessary, but some parents find it comforting and helpful to hear their baby during night and naptimes. Remember babies are noisy sleepers, so not all the noises you hear are them waking up and asking for you!