For a soft, gentle kiss:
* Mwah
* Smooch
* Peck
For a louder, more passionate kiss:
* Smak
* Smack
* Kiss (sometimes written with an exclamation mark: Kiss!)
For a playful, lighthearted kiss:
* Pop
* Boop
For a subtle, barely-there kiss:
* Shhh
* Whimper
Ultimately, the best way to spell out a kiss sound is the one that best conveys the feeling you want to create.