Iridium plugs are advertised as having about half the resistance of platinum plugs [which are horribly ineffecient to begin with], while supposedly lasting about twice as long. Such plugs also are advertised as having overall better performance, while requiring less voltage to fire off. That's pure hype IMO! If you wanna believe in the spark plug fairy, be my guest, but that's 'blue sky' thinking as far as I'm concerned...
The performance and voltage benefits described in iridium plug advertising can be realized using a better set of spark plug wires, than what came stock from the factory, hence my Magnecor recommendation.
Why the hell would someone waste money on iridium plugs without installing better spark plug wires in the first place? And, if you get something akin to Magnecor 'cable' quality, why the hell would you need iridiums?
See my point?
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' phases.
All the improvements, so called, have been to the electrode TIP. Nothing has been done to the ground electrode. So, what we have now are spark plugs with wonderful 'new millenium' plugs with WELDED TIPS that last forever and sh!tty ground electrodes that erode just like they did in 1920.
What's the best IMHO? Generally speaking, for most ppl, that would be the 'tried and true' copper-core spark plug, i.e. the NGK V-Powers. 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 welded tip 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 of the 'oil change', but your engine will ALWAYS be running at peak performance levels.