What is the best exhaust upgrade for the FZ1 Fazer?

It’s not uncommon for FZ1 Fazer owners to upgrade their exhaust as a mod to improve sound and maybe even performance. There are a lot of choices out there, from slip ons to full systems, each with their own flavor of sound, weight savings, and power gains. Riders often argue over different brands and styles of bells, and the discussion often includes price, quality, noise, and how much of an effect the bell has on the character of the bike. Some want the loudest pipes they can have, others a subtle improvement in tone. It also discusses whether some exhausts need ECU remapping or fuel controller installation for best performance.