to OCF !
coretemp and tat only read cpu temps.
I assume the evga mobo comes with a utility that will read all temps including mobo ... so i would use it and calibrate the others against it.
I can't stand the gibabyte utility so once i calibrate it with core temp I only run core temp.
On my asus mobos i usually run both asus probe which has a good screen footprint as well as coretemp since asus probe is very flakey on cpu temps. Old versions of asus prob had a nice grahing feature but it is gone with the newer versions.
The utilities that come with the mobos are proprietary so they only run on that vendors mobo afaik.
Some people prefer the tat intel utility to core temp but i like coretemp's compact design so it doesn't clutter my screen.
What ever you like and gives you repeatable and consistant results.
Absolute accucracy isn't that important, but relative and repeatable accuracy is so you can measure the effect of changes.
sometimes i also try speedfan during initial calibration but it is pretty buggy on new mobo's and cpu's on default setup.
There may be some more sophisticated tools that you can buy, but everything mentioned here is free.