Kwaa, buhh, phjjj, and nnnnggggg are the ones that come to mind right now.
pg
no I jest
pfffft
しゃぶしゃぶ
I would do some pretty despicable things for some right now.
Spakoom! Spakoom! SPAKOOM!
I like the onomatopoeia they have in manga that doesnt really make sense as onomatopoeia because these things dont make sound.
eg. blink, turn, surprise
Kefka's coming, look intimidating!
Have a nice day!!
bonk (being hit on the head with something wooden)
spang (being hit across the head with a frying pan)
paff (the sound bubble gum makes when you blow the bubble too hard)
yiff yiff yiff yiff yiff spluff (I'll leave that one to your imagination)
Poof!