First post has my bet because Mega Headphones sound awesome! Mega Pocketwatch sounds classy af too. Korrina's Mega Glove is cool because I'd like to think activating it would mean clapping your hands together like the alchemists do in FMA. Lysandre's Mega Ring gets a special mention because I'd like to fistbump my Pokemon before they Mega Evolve.
Now, I'm not sure if anyone's mentioned it, but I think a Mega Dowsing Pendulum would be pretty sweet in reference to Lyserg from Shaman King. Of course, it would only look cool if you could manipulate it the way he does with his Guardian Ghost, Chloe. Still, hypothetically, if you COULD control it the way he does, the Pendulum would then make a design in the air corresponding to each Mega Pokemon differently to activate them.
I'll throw a Mega Badge/Medal into the mix of ideas too. That might be more fitting for the police officers, though my proposed way of activating it is to smack it. Ideally, I'd have it on my chest and hit it with a closed fist then throw a peace sign after for coolness points, though I guess it'd be up to the user's preference as to how they'd want to activate it.
STILL THOUGH. Mega Headphones. Like wow. I'd love a pair.
SCENARIO: I'd just be chillin at the park or something, then a group of thugs come by and wanna get rowdy.
Keep cool, calm, and collected.
I'd grab a Pokeball from my belt and summon Medicham, then go through my PokeNav and start playing World's End Rhapsody by Nujabes
*closes eyes and takes a deep breath*
I make eye contact.
"Let's dance, kids."
-bumps side of headphones and watch the sparks fly-