Everyone knows Descartes old experiment now.
He concludes with absolute certainty that “I am, I exist”. However this could very well be a delusion, since the evil genius could have corrupted the mind to think that, or there a theories creating doubt like “we are all one”.
I feel like a more bulletproof argument is: “something exists”. Any form of skepticism, doubt, delusion, incorrectness or falsity around that statement is itself “something”, and so it literally can’t be denied, since denying is something.