OK, this is one topic I'm a biggie on and something that comes in handy frequently.  

Macro answer is it depends on which vehicle and what time of year.  

I have a number of vehicles.  My Diesel offroad pickup is my designated rescue vehicle.  It gets the full Monty from live in the truck a few days to repair about anything that goes wrong.  List is too long and I'd probably still miss a few things but very similar to these other guys lists.  Honorable mentions is shovel, tow straps, heavy duty line, light duty line, pack stove, food, water, and a variety of tools including save all duct tape and very important tie wire.  

Common items in all my vehicles even my motorcycles are tire puncture repair (you learn this living in the mountains with no cell service), shovel, toilet paper, water, first aid kit, lighters, lights, knife, paracord, garbage bags, Mylar blanket, shovel (yes even my bike a small folder),  and tool kit.  

Winter adjustments I add in four wheel vehicles a plastic container of ice melt and in my truck bed two bags of sand.  

Road trip adjustments, I throw in my "Go" bag which man that's a list all in its own but the general idea is 3 days all weather.  That can transfer vehicle to vehicle.  That also includes self defense, yes, a gun.  Gun is a tough one because some places you go you are foolish not to have one and others you are foolish to have one so have to revert to National Car Carry.  Some places you obey the laws and some you are forced to ignore them but pretty much you can stay legal without walking through a land mine field.  

Scenarios you best address are fixing a flat in the rain along side a fast moving road, stuck in the mud or snow, and this broke.  I can't stress more highly the importance of tie wire.  It can be used for millions of things from holding up an exhaust to replacing a clamp.  

Last but not least, any vehicle prep unlike a "Go Bag" should be a living thing with constant adjustments.  Add as you find so to say and often little things become very important.  For example, a multi-tool often comes in handy and you can go inexpensive on a leave it in the vehicle just in case.  Twice I've had my own vehicle catch on fire not even counting other vehicles I've stopped to help.  Glue comes in damn handy sometimes.  

If I had to name the number one item I've used in my life its a First Aid kit.  Due to where I live, I probably stop three times a year for a motorcycle accident which can be very hairy in an area with no cell service.  Now I'm not going to attempt surgery or BS like that on the side of the road, but often you have to address the patient while someone goes to find a hot zone to call 911 and response times push out to over an hour.  Usually someone else will stop to help.  I use these things from everything from a scratch to stop/slow the bleeding. Another thing I do, very important, is pend and pad.  I write down anything I did, tag the patient, for the emergency response team/later doctors.

Number 2, flat tire stuff, pump, plug kit, Slime.

Number 3, stuck stuff, shovel, salt/sand, tow straps.  

Number 4, line like paracord or climbing rope or ratchet straps.  Amazing how many folks are stopped because "That things falling off".  

Number 5, water.  Getting stranded in traffic to need to wash this off or cool this down.  Big fan of the corrugated bottles of water which even in winter can be thawed when needed.  

Number 6, believe or not a biggie in the south is tow straps.  I switch to my 4x4 in snow or ice and have pulled more folks out of a jam than I can remember.

Another honorable mention is I have a Gold AAA card which will tow a vehicle 100 miles.  Comes in handy about three times a year if not me, someone I know.  

Customize your load out not only for time of season but your most traveled geography.  This can vary a lot depending where.  For example when I lived on the ocean, I hauled scrap lumber for wheel traction in the sand.  Not so easy to find stick etc. on the beach.  You get the idea.

What I can tell you is this is one of the preparation things I have probably used more than any other thing we discuss on this forum.  I probably haven't used a spare tire in decades.  In fact, my new Challenger RT doesn't even have one and it doesn't bother me in the least it doesn't.  

Good luck.  Just keep in mind, all you need is a starting point and just keep adding.

Tj

The thing to remember about trailer security is if they have a way to cut a lock, it doesn't matter how many locks you use.  

The two things I do do is I lock both the trailer receiver and truck hitch and use truck hitch locks which are thicker and harder to cut.  

Just keep in mind that unless they're dumber than a rock, they aren't going to be pulling a trailer down the road by hand.  They're going to use a vehicle.  They may push it so far to reduce sound but they will hook it up to a vehicle.  Like all personal property security, its not to stop them but make it so they go to the easier target.  

You know the best part of a security system?  Its the big sign that says "I have a security system."  

Tj