Safest from a data loss perspective or safest from a code getting stolen perspective?
Getting stolen, look at File Vault - it's built into OS X and so fully integrated into the OS. Apps won't know it exists. You can also encrypt Time Machine backups.
Optionally, create an encrypted DMG via Disk Utility and keep sensitive code in the DMG.
From a loss perspective point of view - get an external hard drive with at least Raid 1; encrypt that and chain it to a desk / put it in a safe.