The best is, of course, the one I spent a lot of hard earned dollars to purchase. :-D Same for oil and filters :-D
Some would say the absolute best is the avenue of the least dollars...this would be cutting out a large portion of the airbox and purchasing K&N cone filters to install + a filter for the crankcase breather hose + a cheap aluminum turnbuckle from your local hardware store for a carb brace. There are directions available for doing this job.
Either way, you will need to rejet and setup the carbs for proper air/fuel ratio. The kits come with jets that are close for your particular bike with the aftermarket silencers you have installed. You have installed more free flowing silencers, I hope. If not, forget about removing the airbox until you get the more open mufflers--would be a waste of money, IMHO. Once you have the bike close to the right jetting, I recommend a dyno run with EGA to get the air/fuel mixture correct for all three carb circuits (pilot jets, needles, main jets).