The Allertor tops it for me. The sound output horn just makes it look bazaar. The Banshee is another weird one. Looks like a large pilgram hat. And of course the Darley sirens like the 5/7 and the 2 because of the rounded cowlings. I won't even mention what a Darley 2 looks like
My 1st pick is the Allerter. The projector when viewed from the side reminds me of either a cobra or ET's head, lol.
2nd pick would be the modulator. It reminds me of deep plates turned facing towards one another, or spaceships.
The 3rd pick would be the HOR.
Honourable mentions go to the Crysler Victory siren.
I would have to say that the Allertor is the most unusual looking siren.
I've only seen/heard clips of them on internet sites. However, the city of Atlanta used to have a number of these. I believe they were still gasoline powered, even in the 70s.
There was a red one atop the Hurt Building downtown; it was nicknamed "Big Myrtle." I know that Big Myrtle was taken down many years ago.
I think Modulators look like flying saucers stacked atop each other.
Never seen a Darley for real, so I can't comment on them.