Typically, the lower the ISO the better, but it doesn't always work depending on your other settings and lighting, just as Regric said. Learning about my camera, I only shoot in manual and I take a picture with every possible different setting I can. For instance, I set the ISO and then take a picture at every possible f stop, then vice versa. This way I can see what the settings actually do. If you've noticed, your computer should tell you what the settings were for each different picture too, this helps identifying the differences between what each setting actually does.