There’s no single device which is best for every person. It depends very much on what use cases you are trying to solve. Some people will use a lot of motion sensors, some people won’t use any. Some people find a smart lock is their favorite device – – others won’t use one at all. It really comes down to what specifically you want to do and why. 
You might find the following thread interesting: it’s about the top 10 things to do with SmartThings. ( this is a clickable link)