Is a 9mm the best home defense handgun?

For most people, yes. A 9mm gives you high capacity, soft recoil for fast accurate follow-ups under stress, and — with quality JHP — terminal performance on par with bigger calibers. Get a full-size or compact 9mm, add a weapon light so you can identify a target, load a proven defensive load, and store it secure but accessible. A handgun you shoot well beats a hard-kicking one you do not.