Well, I was talking about the northbridge, but after squeezing the tip of the probe under the waterblock I would say the Intel Thermal Analysis Tool is definitely the one to use. It jumps around 2-3° outside of actual, but was dropping at the same rate as the thermal probe. At load, Speedfan is about 5° below this and Coretemp is about 5° above this, but they both seem to be reading the diode. Coretemp is much closer at idle, though.