I use Clorox toilet bowl cleaner and a toilet scrub brush.
Heat up 1 cup of white vinegar, mix in about 1/4 cup of Dawn Premium. Not the dollar store kind, because it is too thin. When mixed, pour into a spray bottle and fill with water. You only need to heat up the vinegar one time. Spray your toilet let stand over night. then rinse with a toilet brush. If stains are stubborn use a little Ajax or Comet.
I pour 2 cups of white vinegar in the bowl to soak over night or all day if we will be gone to remove the minerals. Then I scrub it out with a brush. If there are any stains still stubborn I can soak it again, OR if in a hurry I use Mr. Clean magic toilet brush that does a great job but is a little expensive. Finally, to sanitize and really polish it off I use a non toxic cleaner called Simple Green. It is a disinfectant and a deodorizer and I use it everywhere in the house. You can get it in spray or concentrate in home improvement stores or at Walmart in concentrate.
As I have a water softener, ion exchange type that regenerates with salt, the ONLY true water softener, I have little issues. But a sort of slime will develop. I have a retired medicine eye dropper filled with straight bleach. The last flush every night I add about 5 drops to the bowl. It stays IMMACULATE.
Pumice stone for rust and hard black gunk
I pour bleach in bowl and let sit then clean with the brush