Aperture mode or Shutter mode

Sometimes you want to control for depth of field -A mode-

Sometimes you want to control for motion blur -T/S mode-

-Edit-

Also, you usually don't want to adjust ISO to fix over/under-exposure. Shoot to your camera's native ISO unless you literally don't have enough light or your exposure is super off. Try to use the lowest or closest to native ISO as you can, and use the cameras built in metering adjustment feature (e.g. if you're under exposed tell it to over-expose by a stop or two...)