How can I get rid of water stain in toilet bowl.

Wow, that has to be driving you crazy. Is it possible that it's not a stain, but that someone tried to get a stain out with sandpaper or something else that is abrasive and it left that mark? You could try filling your bowl with vinegar which would remove hard water deposits. But if it's a stain, it may not help. You didn't say if you've tried bleach. It's a pretty harsh chemical so I don't know if you want to try that. There are also a lot of porcelain stain removers on the market, but you know that if you've tried several. Have you tried a rust cleaner? Maybe it's not just a regular stain but is rust, although if you have a whole house softener it should clear the rust. I wish I had more to offer, but my thought is if none of these work, it might be damage to the finish. Sadly you can't clean that away. Wishing you the best!

What kind of stain? rust or lime? if its rust, check the tank, it usually comes from there.... put some CLR/vinegar in there and then use Bar Keeper's friend to scrub of the stain after a day.

Dollar General it's called The Works

Go to Dollar General and get The Works. Also try Tilex.

Try soaking it with white vinegar over night (2 cups or more) and then I also use Mr. Clean Magic toilet brush. It has the same cleaner as the Magic bar cleaners and really removes stains.

have you tried a pumice stone?