The plugs the factory uses are made by Champion, but with Harley-Davidson printed on them. Go to Wally World and save a few dollars. Use the cross reference. I spend about $1.25 each at Wally World versus $3.00 each at the dealer. Mine don't have to have Harley-Davidson printed on them!
I haven't changed them in my bike yet, but in my cars I have had great luck with NGKs. Also, a buddy of mine with a sporty fouled out several plugs and then put in the SE ones ($6 ea ouch), but he hasn't fouled those. So I guess it was worth the price.
I always put a dab of anti-sieze on the threads before installing new plugs. Screwing steel into aluminum that get's hot can cause problems. A tube of anti-sieze from the auto parts is a few bucks, good insurance against a stuck plug.
I do not have them in my bike yet. They are on the way though. I have had great luck with DENSO Iridum plugs in my jetskis and atv's. DENSO was the first for Iridum plugs. All the super bikes use them.
For the TC88 the Denso number is IXU22
For the 1200 Sportster 96 and up is Denso # IXU22
For the 883 1985 and up again the IXU22
The 1340 from 1985 and up uses Denso number IW16
That is from the Denso Iridum plug guide.
Did you know Denso makes parts for your big twin? They do, here in East TN.