Spark plugs have been around for over 100 years and they haven't changed a lot. They spark, they erode, they die. The trick is to replace them somewhere between 'erode' and 'die' phase.
All the improvements, so called, have been to the electrode TIP. Nothing has been done to the ground. So, what we have now are spark plugs with wonderful 'new millenium' tips that last forever and sh!tty grounds that erode just like they did in 1920.
What's the best? Generally speaking, for most ppl, that would be the 'tried and true' copper-core spark plug, i.e. the NGK V-Powers mentioned above. They work great, they're cheap, but they don't last very long. The hot setup, if you'll pardon the pun, is to replace them every 15-20k. That'll put you right in the middle of that erode/fail cycle I was talking about.
The absolute best, of course, are silver-core plugs. But, they're expensive, hard to find, and don't come in as many different flavors as copper-core plugs.
As far as which plugs are the most efficient:
Once again, silver-core is the best, but copper-core is a close second.
One thing to mention here is the thermal conductivity thing. One of the most important features of a spark plug is its ability to carry heat away from the combustion chamber. THIS IS MUY IMPORTANTE! And, that's the primary reason you are supposed to use a torque wrench when you install plugs; to allow proper heat transfer. I don't want to put you to sleep, so we won't go any further into this other than to say, look at the chart and you will see copper-core is far and away a better choice than than tiny platinum or iridium tipped plugs.
Hope this helps. Personally, I think Iridium TIPPED plugs are a waste of money. If you're going to 'waste' your money, buy copper-core plugs and replace them at every oil change. That add about $8.00 to the cost, but you will ALWAYS be running at peak performance levels.
