For an electric, the old triple-note Cadillac has a great sound. Mounting position is what matters.
Air horns cannot be beat for sound. But there are limits (highway only on air horns for trucks).
I supplemented my RAM truck horns with the old autobahn standby: the piercing note HELLA pair (see
Susquehanna MotorSports - High Performance Vehicle Lighting and Competition Accessories).