The previous replies about DFU restore aren't wrong, but it is intended more for phones that no longer boot properly. The end result of all three of these methods are exactly the same...a clean install of iOS. You wouldn't be able to tell which method was used.
If you're just trying to reset and remove all data as if it's a clean install, 'Erase and Reset all Content and Settings' is sufficient. It will leave absolutely no trace of your data behind. It'll also make sure you're signed out of iCloud/Find My, so that the phone doesn't remain activation locked.
iOS is installed much like macOS, in its own volume/partition that is signed, sealed, and marked read-only. Your data lives in a separate, encrypted volume/partition. When you erase all contents and settings, it wipes the data volume by removing the key that's used to decrypt it. There's really no need to re-install iOS since it is on a signed, sealed, & read-only volume...nothing should have changed it.
If you do a iTunes restore or DFU restore, then make sure you are signed out of iCloud/Find My first, especially if you are planning to resell the phone...otherwise they won't be able to activate it.
