AC Delco copper or any brand plain jane copper plugs. Not iridium, not Platinum, not split fire. I heard iridium may work but copper is the way to go with these engines and stay as far away from Platinums as possible. It will run like crap with those plugs I can promise you that. It's because of the temperature and distance of the spark why it's best to use copper for these engines. What are you using now with it?
Also at this point since it's been going on for a year I would check your coil pack wire going to cylinder 1. What did the plug look like when you pulled it?