I'm never sure what the best words for sound effects are in comics. I honestly don't care for "pow" for a punch. I'm probably the only one who thinks PAF! is appropriate, but I'm sticking with it XD.... 'paf' might be a french thing.
In any case, its got a lot of 'oomph' to it.
@rek I love PAF as well, it reminds me of Astérix or other classic BD franco-belges.