how to define a user id securely

I have a database whose entity are users.My database is online. When a user registers in the game, we must define a user id for him. id username password coin score ... do we need player unique identifier "SystemInfo.deviceUniqueIdentifier" for id? or we can easily use concequtive id from (one two three ...) or special number like 100000,100001,...orderly. Please consider a secure method to prevent hack.