Best martial art for law enforcement? Gunjitsu. Definitely, Gunjitsu!!!

But, seriously folks...
I've dabbled in all kinds of martial arts over the years, and seriously pursued Shotokan for many years. When I was in the Army, I was a hand-to-hand combat instructor, and, since becoming a LEO, have been certified as a law enforcement defensive tactics instructor (PPCT) amongst other things. I've trained in several
extremely aggressive close-quarters combat systems that would
not be defensible outside of a true combat/"life-or-death" situation. Does any of this make me an "expert"? By no means. But I feel I'm qualified to have an opinion, at least.
My take, for what it's worth, is that most formalized martial arts are just too
complicated to be useful on the street. Aikido, in my humble opinion, falls into this category. I mean, if you studied most of these martial arts for years and years, hours a day, you would almost certainly be a certified Bad Motherf****r. But, be honest, how often do most LEOs train on defensive tactics after finishing the academy?
Grappling and ground-fighting techniques are great... until another bad guy shows up and starts using your head as a soccer ball. (Ask me how I know this...

)
So, what do I recommend? Something simple; direct; brutal; something that instills a few, effective techniques that can end a fight quickly and can become part of muscle memory on the basis of repetitions. No flying-spinning-cartwheeling-back-kicks, complicated joint-twisting, or other "Hollywood" techniques. Something that emphasizes staying on your feet if at all possible, but incorporates groundfighting if it becomes necessary. From my experience,
Krav Maga fills this bill. I've trained in KM for a while now, and I think it's great.
Just my opinion, your milage may vary....