Which NGK/Denso plugs do you guys use for long life/optimum fuel economy? Which are best: regular copper, platinum, double platinum, or iridium?

 

Assuming the 1998 engine uses the same plugs as the 2001, per my Owner's Manual, you should use either the NGK PZFR5F-11 or the Denso PKJ16CR-L11 plugs. Anything else will be a crap shoot regarding reliability, suitability for your engine (from a operational smoothness and power/fuel economy perspective, not to mention longevity).

 

I like to buy the same brand that was installed in the car. Denso or NGK are good brands. Just make sure they don't sell you a set with a different number or letter on the part number.