Whats the best sensor to buy for Pet owners?

As @rpittser points out, in addition to the inexpensive “pet immune” sensors, which typically just have two or three settings, some of the more expensive motion sensors like the Fibaro have a much wider range of sensitivity options. So again, combination of sensitivity setting and positioning may enable you to ignore pets. But the details still matter.

Someone, maybe @Mike_Maxwell ? Uses motion sensors embedded in the wall with a narrow tube over them to really limit the detection area. Set at just the right height these can be very useful for distinguishing adults from some pets, but again may not be able to tell the difference between a small child and a large dog.