Wipe Android phone before selling What is the smartest way, before selling a mobile phone, to be sure that no one will be able to access my data?

In my case we are talking about an LG G4.
A factory reset is not enough. It just "disables" your data and doesn't really delete it. Some data restore tools can still restore the data after a normal reset.

The best way I know of is to encrypt the phone and then do a factory reset. The encryption will overwrite the unencrypted data, so that will be gone forever. Then after a reset, if the new user tries to restore the data, it just gets the encrypted data. (=useless)