Common:
* Mwah
* Smooch
* Kiss (This one is more of a written description than a sound effect)
More playful:
* Mwuah
* Mmwa
* Muah
* Pwah
Descriptive:
* A soft, wet sound
* The sound of lips meeting
Ultimately, the best way to spell out a kiss sound depends on the context and how you want to convey it.