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!
None of Them! I have never been a big fan of baby monitors, unless you live in buckingham palace or something. Here is why... They do not prevent a baby from dying from sids. They inhibit the natural process for babies to soothe themselves and get themselves to sleep (goal-between four to six months of life.) they foster unnecessary anxiety among new parents. http://healthergy.net/groups.