Pepper spray or gel
The default recommendation, and mostly a fair one.
Inflammatory spray. Forces the eyes shut and the airway to constrict.
Carrying it legally
No permit in most US states, though several cap the size, set a minimum age or restrict the formulation. Banned or tightly controlled across much of Europe, Canada and the UK.
Air travel
Prohibited in carry-on. Checked bags take one 4 fl. oz. container with a safety mechanism, under 2% CS or CN by mass.
Where it fails
The wind turns it back on you, you are inside a car, or the attacker is already past arm's length. The effect is also not instant — expect a second or two.