What martial art would be a good one to learn if I want to be a police officer?

I myself do Taekwondo, but I am not an officer.

There are many good martial arts and self defense systems. If I had to choose, I would recommend Krav Maga or Brazilian Jiu-Jitsu.

Brazilian Jiu-Jitsu Definitely

Any would be good to learn. But you would want to make sure that if you used a martial art technique while on duty, that is was permitted per your department's use of force policy.

Brazilian Jiu-Jitsu is definitely the best

KRAV MAGA!

As a police officer, you have to follow your department's use of force policy. That being said, I think that without question, Brazilian Jiu-Jitsu is best for police officers.