I imagine any pipe with a removable baffle would the the "loudest". I pulled the baffles from my Specialty Spares long cannons and they are CRAZY loud. As in "neighbors come over and tell you about it loud", set car alarms off loud. Actually I set a few alarms off now and then with my baffled Cannons, albeit with 30% reduction in the fiberglass packing material. I can't imagine anything being as loud as straight pipes. I guess you could completely remove the baffles in a stock pipe (snekpete did) and they would be loud. The 3" echo chamber of the empty Long Cannons would be louder I suspect.
OF course therein lies your dilemma - straight pipes don't offer the same performance as pipes with back-pressure.
