What is the best exhaust for the Duke 790?

A lot of Duke 790 owners want to upgrade the stock exhaust for improved sounds and possibly some performance gains. All of this results in discussion about what the best options for sound quality, weight savings, performance upgrades, and overall value are. Some riders like slip-on options for their ease of installation and cost, while others argue for full systems to really benefit. You'll also find discussion regarding whether certain exhausts need to be ECU-tuned to work correctly and how different options change the power delivery of the bike. There are also noise levels and legal considerations to consider; some riders prefer a loud, aggressive sound while others want something a bit more mellow for daily riding.