Battery negative or positive ground?

Negative is always the ground and is the last cable to be connected. Any vehicles with a positive ground go back to at least the 1950s or older.

The reason we connect the negative last is simply for safety. As you're wrenching away, if the wrench were to contact anything metal on the car while touching the nut on the cable clamp, nothing will happen. If you connect the positive cable last, there is the potential to touch something metal while the wrench is in contact with that positive cable. That would cause a dead short, a real lot of sparks, and I've seen a wrench become welded in place and turn orange red until the battery was dead. We don't need that kind of excitement.

Similarly, we also remove the negative cable first. Once that is done, if the wrench contacts something metal and the positive cable at the same time, nothing will happen. If you remove the positive cable first, there's that potential again to create a dead short.

Be aware there will be two negative wires in the cable clamp. The really fat one bolts on the other end to the engine or transmission. It carries the very high starter current. The second wire is much smaller, but still larger than most of the car's other wires. At the other end, it bolts to the body sheet metal. That's for the lights and all the other electrical stuff on the car. The engine and transmission are mounted on rubber mounts, so that large cable bolted to the engine won't work for the rest of the electrical system. You must use both.