For me Speedfan works best.
It shows the temps of my HDD my CPU my GPU and etc.
It also tells me how much of my CPU is being used.
It can also control your fan speeds if your fans have 4 headers and are hooked up into the motherboard.
Pus it tells more accurate temps then Rivatuner, which was designed JUST for the GPU temps.
So go with Speedfan, I recommend.