Simple:
* Hiss - This is the most straightforward spelling, conveying the basic sound.
* Sss - This emphasizes the sibilant quality of the hiss.
More Descriptive:
* Shhh - This adds a slight breathiness, suggesting a quieter hiss.
* Hsss - This emphasizes the sharpness of the hiss.
* Phsss - This adds a bit of a throatiness, suggesting a more guttural hiss.
Onomatopoeic:
* Hiss-hiss - This replicates the repeated nature of some hisses.
* Pssst - This captures a sudden, sharp hiss.
Ultimately, the best way to spell the sound of a cat hiss is the one that best captures your impression of it.