Splitfire? I call them Shitfire.
I use NGK's or Autolite's.
I also use colder heatrange plugs, DPR9EA9 NGK or DP4162 Autolite, but that is a decision you have to make. The equivalents for the stock HD plugs are DPR7EA9 and DP4164. There also are in between plugs 8's and 63's but they are hard to find.
The reason I hate HD plugs isn't price, but they are Champion's and they have a habit of tearing up threads in the heads. The NGK's are the best in this respect because the threads are rolled and their coating is very good.
Some Delphi bikes will set codes with some spark plugs, and will definitely set codes if you change plug wires.
If you are building a really hot engine you can use the Denso Iridium IXU22's, but they are not cheap.
Hint, the 9NGK is a GoldWing plug, LOL.