on rennlist over the years, various people dyno-tested dozens of plugs to get a definite answer, and NGK's consistently made more HP. every time a 'new and improved' plug is introduced (like the bosche platinums, which are a scam), someone always back-to-back tests them against the ngk's and none have beat it yet.
there is actually one plug that is the king of them all, which are the ngk pure platinum racing plugs, which have a solid platinum core electrode (not like the needle-thin wannabe electrode bosche tries to pawn off), but at more than $50 per plug, they're not really worth it for the small performance gains;
http://www.ngksparkplugs.com/products/racing/racing.asp?nav=16100&country=US