Memories of Murder is his next best; all of his films do a phenomenal job of shifting moods, no matter their main premise. Parasite is the perfection of a formula, but he's been toying with it for years.
Memories of Murder
Parasite
Mother
The Host
Snowpiercer/Okja
Tokyo (this is a little different since it's an anthology)
the only one i haven't seen is his debut. Parasite is definitely his most accessible (i guess Snowpiercer too but it really is not like his other stuff).
there's an Every Frame video that breaks down one of the scenes in Memories of Murder that's a good watch too