The "best" exhaust completely depends on what machine we are talking about. For the 400ex, the absolute best full system exhaust is the sparks. For the trx450r there are several top names, but FMF unfortunately is a terrible pipe for the trx450. hmf, sparks, dasa, rossier, are all top pipes for the trx450r. As for slip ons, you're not going to see huge power increase so there are generally very little power differences between slip on pipes.