Not necessarily the best nor even my favourite intense movies. Just the first ten I could think of.
Irreversible: One that doesn't get a lot of viewings because it only really needs the first. The point to the reversed timeline is the eradication of moral identification. It works. It's tough because that works. It's extraordinary.
The Exorcist: Friedkin approached the genre piece as though it was a true story. All the generic traits were discarded, recalling that empathetic pain and fear are most effectively related by making them look real. They do. THEN you get the mystique and dry ice horror because then the alienness of it also feels real.
Repulsion: Catherine Deneuve is driven insane from fear and we're in the back seat. Polanski still had a few gems to make but he never topped this for intensity.