The personal identification number (or password) is the most tried-and-true and simplest form of local security. Most users opt to protect their device with a PIN that is at least four digits in length, while some go for a longer, more complicated password that combines both letters and numbers.
Those who care enough to make their PINs/passwords long and complex will enjoy a greater level of security here. After all, this option's greatest weakness stems from user error (or rather, apathy): lock your phone with an easily-guessed password like "1234" and that's precisely the level of security you'll enjoy. (Also see: Basic password hygiene)