Look for a large luxury car or a full size truck. Crown Vic/Grand Marquis. F150. As mentioned elsewhere, decibels are limited by power, so look for something with a lower pitch. Lower pitch carries farther, so it will sound louder even if it isn't louder, if that makes any sense. Low pitch also means "big vehicle" so there's that mental effect at play.

Take both horns, the semi-dissonance will also help.