* "Toot" is probably the most widely used and recognizable spelling.
* "Beep" is another common option, particularly for smaller, more compact horns.
* "Honk" might be used if the horn is longer and makes a more drawn-out sound.
* "Bla bla bla" or "Poo poo poo" are onomatopoeic spellings that capture the repetitive nature of the sound.
Ultimately, the best way to spell the sound is the one that best represents how *you* hear it.