Any jumper cables at all. Most people don't carry them anymore or practice safely using them. Or are too nervous to hook them up. They worry about being shocked or shorting the battery.
Get some reasonable quality ones that can still fit in your spare tire well. Cables that are too long generally get left in the garage if they don't stow away nicely.
Do yourself another favor, and get a small plug in electric tire pump. Saves you having to stop by the service station to add air to your tires. Also, if you have a slow leak it can get you to point B.
If your spare tire is flat from not topping it up, you can use said air pump to pump it up if it is needed in an emergency.
Watch some YouTube vids on how to jump a car to do it safely.
Positive to positive, then connect neg on battery post of running car, then connect neg LAST on engine or tranny of car that needs the boost. This is to prevent sparking near a battery that may be leaking Hydrogen gas and may explode.
To disconnect, unhook neg from engine of boosted car first, then neg from car that performed the boost, then the positives and put jumper cables away.