I have been using a foaming Clorox toilet cleaner and it works great with a toilet brush. It also smells amazing unlike a lot of other cleaners.
Sprinkle in baking soda, and and then squirt in white vinegar and scrub.
I use Dow Scrubbing bubbles toilet bowl cleaner with a toilet brush.
CANNED COCA-COLA. Pour you some down inside the bowl & wait overnight. Scrub with the toilet brush & flush. If it all don't come out, do it again.
Go to Home Depot and buy a Pumice Stone, they are made to remove hard water stains in your toilets. They are about 6 inches long and 1-1/4" X 3/4" square. Wear a pair of rubber gloves and place the stone in the water, then rub in on the porcelain and watch the stains go away. Use it once a month to remove any new stains and your toilet will look like new. They cost about $4.00 Pretty sure only plumbers know about them.