Informal:
* Spat - This is the most common and informal way to spell the sound.
* Spit - This is also common, but less specific to the sound itself.
Formal/Onomatopoeic:
* Pfft - This is a more onomatopoeic spelling, capturing the sound more directly.
* Tsk - This is another onomatopoeic spelling, often used to describe a disapproving sound.
The best way to spell it depends on your intended meaning and the tone of your writing.