What is his best film?

Children of Men

Children of Men by far.

I also say "Children of Men"

I'm going to fly the flag for Y Tu mama Tambien, that film deserves a lot more love. Great film, plus Maribel Verdu as one of most underrated beauties, damn she's smoking. Bernal and Luna have great chemistry together.

But Children of Men is a classic. Still think Gravity is overrated, probably that's just me.

Y Tu Mamá También followed by A Little Princess.

I really liked Roma, but I have yet to see Children of Men. I liked Gravity too.

Gravity
Roma
Y Tu Mama Tambien

I began a fan of his two decades ago, my answer is alwasy the same in 20 years: A Little Princess.

It's his first movie, and still the best movie.