30+ yrs ago there used to be a CHAMPION Spark Plug plant in the NW corner of the intersection of US Rt 34 & 61 in Burlington, IA. Not sure what's there now, been 25 yrs since I was through there last. I'm sure Champion had other plants too. They're now part of Federal-Mogul, and I think they're a better plug than they were for a while, probably as good as they ever were.
The last set of plugs in my Super H FARMALL I installed about 30 yrs ago, and haven't quite got my 250 hours on them yet. Last fall the SH got hard to start for a while. Thought plugs would be a good place to start, bought a new set of Champion D-18Y's to replace the old D-15Y's. They worked like a charm! With the new box of plugs still sitting on the pass. seat of the car the Super H starts right up now! Guess maybe starting the thing every 3-4 days to push snow helps! Dad put a set of D18Y's in the '51 M back in 2005 and it's maybe run/idled ten hours since then, it hasn't run since September/October and if I went out and started it I'd fire up on the second or third revolution.
The old K161/181, whatever it is, in the 70 was hard on plugs, burning stale 2-stroke pre-mix, idling a lot, and the carb inlet needle would stick if it sat for more than a week, fouling the plug when I finally did get gas in the carb from over-choking the engine. I tried Autolite, NGK, and Champions seemed to last the longest.
The thing I have against Autolite is they recommend the SAME plug as the replacement for the Champion D14, D15Y, D16, D18Y, D21, and D23. How can ONE plug properly replace SIX? Just because the thread & reach are the same doesn't make it the right plug for ALL applications! The D16 was the plug for most IH engines, also Allis, and a couple other brands. The D15Y was a cooler plug but the extended tip put the spark further into the combustion chamber for more heat. It must work because the Super H will idle for an hour at Zero degree temps and not foul a plug.
The J-8C and H-10C, like Brain W. said, is a Copper Plus plug, has a copper core on the center electrode for higher heat dissipation & better conductivity. When the "C" starts the plug number it means "Compact" like for use in chainsaws, string trimmers, & leaf blowers. There was an old "Commercial" line of plugs that had a "C" suffix but they only had two numbers and the "C". And Champion also makes a series of plugs for race engines that start with the "C" prefix.
There may be better plugs out there for some applications, but NOT for $2-3/plug.