Security expert Tarah Wheeler recently tweeted a list of the top 20 most common iPhone passcodes (via Cult of Mac).
Top 20 Passcodes
If your passcode is on the list, you should consider changing it. These are all four-digit codes, and Apple made six-digit codes the new default, which are more secure.
You can change your passcode by going to Settings > Touch ID/Face ID & Passcode > Change Passcode.