I suggest you replace your headpipe, or remove the cat first. Your motor comes from the factory running too hot and lean. "Best" is subjective. Are you concerned about performance, looks, tone, or dB level? You'll get minor improvements in all those categories if you replace the headpipe FIRST. And the motor will be running cooler. That means longer engine life, and comfort while idling.
Nobody's going to agree on the "best" slip-ons. If it's just LOUDER you're after- and don't care about performance, you can gut the baffles and have straight pipes. Doesn't get much louder than that. And when you open the exhaust end, you need to open up the intake as well. Then flash the ECU for your modifications. We can't get by with re-jetting the carb, and adjusting the timing by ear anymore.
Rather than posing your question to the masses, you can search thru the exhaust section of the forum. Your questions have been asked and answered thousands of times already.