I haven't changed mine out yet, but in the manual it lists two different ones, a NGK and I cant remember the other one. I went to buy a few at the local NAPA, but they had to special order them. I've had the stock one in there since august 08.
my factory plug fouled out after 2 or 3 rides. I was kinda concerned, but my dealer told me it was normal due to shipping materials. so, I replaced it with the NGK and put probably 50 hours on the bike and haven't had any more trouble yet. hopefully my dealer knew more than I thought. I thought he was a complete idiot. I've never heard of such a thing.
Oh, Yeah, I asked them about a different plug, such as split fire or some other higher quality plug. they said " nope, don't have it, can't get it, don't think they make it."
I have replaced mine about 3-4 times for tuning it. I just get the ones honda has in stock.there not that bad priced I think like $ 5-6 each. not to bad considering some people change there oil after every ride.
I would stay away from the Denso. Go NGK all the way. I had a denso plug in when it broke (didn't foul) at the Parker 250. The plug had a hairline crack at the base where the electrode sticks out from the plug.