What is the best way to make my 2015 Yamaha r3 motorcycle way louder?

Why did you not say 300cc 4 stroke?  I had to look up WTF is an r3.   That is irritating.

. I had a Kaw 400 twin 4 stroke

. I had Chrome Shorties welded on so they were beside the rear wheel aimed toward the pavement as I had added curl tips. (car exhaust tips) So basically got rid of most of the muffler so it ends under the engine.  Dual exhaust like sidewinders on a car.  Onto that was welded with stainless steel weld the chrome shorty and the curved tips were added either screwed on or welded to the other end of the Shortie.  They will go by various names but is a short straight through muffler(basically barely a baffle) and aiming it toward the ground has the sound bounce off the pavement into YOUR ears(it is you you want to impress)  They can be quiet and they can be loud(depends on the amount of throttle you give it)  It sounds like a Triumph or BSA or Norton kind of.   People on the road can hear you coming when you want them to.  It is loud enough otherwise the cops will pinch you for noisy bike.

Back in the 60's, they had a device called a 'Snuff or Not'. It was a sort of silencing device that the rider could turn on/off with his fingers. That way, people could get a good night's sleep. However, I don't know if they worked all that well.

Paint it bright pink. That should get you noticed, without disturbing people’s sleep.

Fart as you ride along.

Louder. hmmm. isnt performance more important? Dont get me wrong most of my rides are loud but they still enhance performance. Assuming you already have a decent 4 into 1 exhaust all you need to do is remove the baffle. Cut it in half and put it back in. Cut out small section in the middle actually . then weld it back together. You get plenty of sound and not hurt bottom end torque. Or get a super trapp exhaust . its tuneable. Has removeable plates. Its the only exhaust made that you can tune like this. 

Just remove the exhaust, since ya don't care about power output or burnin' up them valves.

Change the exhaust system and compensate in the intake. Look for what Hyabusa offers. Get more performance along with the sound.

talk to a mechanic who knows exhaust systems, no guessing.......maybe ask the Yamaha dealer.....

Take the exhaust entirely off.

Ride without a helmet. That will make it so you can hear the exhaust better.  Maybe try riding with your head closer to the exhaust.