It really depends on the overall legal climate. For example, as I understand it, in the UK, you can be prosecuted for possessing or carrying literally any item if you're doing this with the intent of using it as a weapon - it does not matter if you only mean to use it when provoked or not. So, any advice you get here may be still patently illegal.

In terms of practicality, pepper spray is probably the best choice. It's easy to always have within reach, it works against multiple assailants without requiring close contact, and it incapacitates people pretty reliably (exceptions happen).

Most other tools suffer from the problem of requiring remarkable dexterity or strength, or a ton of training, or only working once, or needing you to get very close, or just not being practical to carry around or have within reach whenever you open the door.