You didn't mention legal or not. The loudest would be long tubes with turndowns. You would loose torque without the X or H pipe though, but you would be heard a couple of miles away.
actually my mustang ( 70) had a 302 with Lt's and 2.5" exhaust going into original 40 series flowmasters. It set off car alarms idling, and a guy up the street came down bitchin that he heard it over his lawn mower when we were breaking in the cam.
but now its gettn a 700 horse 431 strokker FE with 3" exhaust and super 40's so itll eb evern louder now....haha