The NGK Powerdex AFX with the NTK O2 sensor is supposed to be very accurate. The big digital readout is easy to read also.
http://www.ngksparkplugs.com/products/performance/afx.asp?mode=nml


Check out the Dorn's 69 Barracuda "Switchblade" in the Nov.2010 MCG
Check out the Dorn's refurbished 36 Ford in the Feb.2011 Street Rodder