OK this question comes up quite a bit so I have gatthered what I can from the different boosted members of the site to help aid members in there decisions.
I use NGK Iridium BKR7EIX-11
and Denso SK16R-P11
I use NGK 3330. there cheap and 1 step colder.
I use NGK Iridium BKR7EIX-11 gapped to 0.28
denso IK20, IK22, or IK24 increasing steps colder. the more horsepower the colder you go. You can also use the stock GS400 plugs which are the equivilent of IK20's. all gapped to .029 or so, can be closer(depends on ignition etc.)
I'm kinda partial to the Denso Iridiums. IK20's for TT and IK24's (2 steps colder then stock) on a good sized single. Downside is price (~$75 a set). If money is an issue the NGK 3330's are OK but it's generally agreed to change them with the oil (every 3K). At $10 a set, they are signifigantly cheaper. FWIW, the Densos can be found locally at Olympic auto parts down on RT193.
Im using Toyota GS400 plugs
I run 98 supra twin turbo plugs from the dealership. Been boosting for 75k and have had no problems with plugs.
responce to the above from Ryan23
The stock TT plugs are Densos, decent plugs... maybe a little too "hot" for a higher boosted single though. The cooler plug will have less of a tendency for preignition (detonation/"pinging"). I have used them in a pinch and they do last quite awhile.
I've got the GS400 denso iridiums in... no complaints.
I'm running IK24s gapped @ .030... they've been treating me very well. I was running the 3330's before but the downside to those is that you have to change them every 3000 or so miles. With the iridiums you'll only have to change them about 20000 miles or so.
Swift Racing Technologies
SRT Plug Choice for any boosted cars:
NGK BKR7EIX, Part# 2667 Gap depends on the ignition system setup.
Do not get the BKR7EIX-11 (Gap is too large to start with)