A Kubaton and handcuffs, just in case.
Great for you @MikeBKY but for most of us if we put handcuffs on someone we might be going to jail for the use of unlawful restraints (language might very but I am sure you know what I mean).
That’s why I do not carry them on my person. In the unlikely event I am going to arrest someone, meaning a citizens arrest, it is going to be a very big deal and I don’t take it lightly. And there is a very good chance they are suffering from recent case of lead poisoning but are capable of becoming a threat again. I am intimately familiar with Kentucky’s laws relating to arrest by peace officers and private citizens.
Israeli Bandage & Tourniquet. Other than that there are a whole host of “useful” items that are in every car I own. For Self Defense I keep my Tire Iron (L shaped one for the vehicle ) between the seat and the door or trans tunnel. Not a big fan of leaving “weapons” in my vehicles but there are places for them should I decide I have the need.
IFAIK, at least one rescue blade.
A fast opening liner lock S&W Border Guard knife.
Too large and heavy to carry comfortably, it fits in the glove box hidden under a road map.
It’s also cheap ($11 on sale at Big Five) but nasty looking,
4.1" tanto blade and a windshield breaker on the end
Just in case.
Two items in permanent residence in the car are a solar powered flashlight w/glass breaker & belt cutter (and other features), in the door pocket and an 8" dagger between the driver’s seat and console. There’s other stuff too, but asked for only one.
I don’t keep anything in glove-box due too my own immaturity @ age 19-ish.
But IF did it would be something small in caliber.
Tissues. Napkins. Pepper spray. Straws. Flashlight.
A couple different knives and high intensity flashlight. There may or may not be a laser pointer type thing as well to help visually incapacitate an attacker.
AR15 pistol holstered in pvc U-bolted to wall behind seats in work van. My black jacket hangs on coat hook above it for concealment. Also keep 1911 .45 down by driver seat with two extra Wilson mags.
“Failing to prepare is preparing to fail”
2 first aid kits one in the trunk one in the glove-box, flashlight, extra fuses, batteries, car jumping kit with extra chargers , extra mag, and my xd-9 in the middle console.
Since you ask for only one thing, I have as an extraordinary item in my console, something that is considered an “emergency sling.” It falls under emergency medical but its really a relatively thin, but strong length of fabric about 4" wide by 48" long. Its kind of a do everything kind of thing. It is folded up very compactly, probably 2" wide by 4" so it takes up very little space. It is meant to tie and hold so it has many uses without being as in your face as driving around with handcuffs or flex cuffs in my console. If you look at the packaging it can be used to tie slings, splints, bandages, bind wounds and bandages, tourniquets etc. Think of it similar to an ace bandage but without the stretch. So, take that to the next level, you can use it for all those things, and as a father of two very young but active boys, I may very well have to use it for those purposes. But if needed, I have no doubt it would hold and I would be able to tie it very tightly to use as a restraint.