If you're parked between cars, it's preferable to just leave it in neutral: this way, if somebody bumps you, it doesn't harm the transmission. Just make sure you've got the handbrake firmly set.
If on a hill, 1st gear if facing uphill; reverse if facing downhill. Just make sure to set the parking brake before releasing the clutch, so that the weight of the car is on the brake first; the gear is there only as a backup should the brake fail.