Best, depending on how that is qualified. There are better, but they cost a lot and the difference isn't really enough to justify it. Most copper electrodes are alloyed with nickel for heat resistance.
Even aircraft engines which operate almost exclusively under constant and immense load use copper-nickel plugs.
I've had a few vehicles that came OEM with iridium plugs and the Mfg in each instance acknlowdged it was because of the increase chance of fouling and not performance or power which required their use. One was a Honda big twin motorcycle with dual plugs, but it was a big bore and the plugs were so far from center that they collected carbon no matter what the AFR was. The others were boxer engines that dealt with higher oil consumption.
I did some reading on plugs that lead me to run platinum ones in my dart, but I really haven't noticed anything. The thing that got me curious was the firing voltage for a platinum is supposed to be lower and so less chance of misfire. But the more reading revealed that the lower breakdown voltage isn't actually a benefit because less energy is actually jumped across the gap and in very rare instances there can be secondary spikes and other electrical black magic that can nuke sensitive ignition electronics (which our cars don't have, but some late model and especially exotic ones do) which is why OEMs are such sticklers for using the exact right parts.
