Answer by taking a quote from the following article:

Krishnan Nair (25 July 1939 - 16 November 1980), better known by his stage name Jayan, was an Indian film actor, naval officer, stunt performer and cultural icon of the 1970s. He starred in over 120 Malayalam films. During his film career, he was primarily an action star and was particularly famous for his macho image and unique style. He was reputed for his chauvinistic appeal and well known for performing stunts of a dangerous nature on his own.

In the late 1990s and early 21st century, there was a resurgence of Jayan's screen persona in Kerala and his old movie scenes came to prominence again. It was owed mostly to programs by popular mimicry stage artists in the State, whose imitations of the star's mannerisms caught on and soon became commonplace in college stage events, television programs and mimicry stage shows along with quotes of superhuman strength known as Jayan quotes. However, it has been pointed out that many grotesquely imitated screen dialogues of Jayan are not actually his, but that of dubbing artist Aleppey Ashraf, who dubbed for many of his characters after his death.  The "comeback" of Jayan and his renewed popularity lately may be taken as an affirmation that Jayan has not been replaced even nearly three decades after his death. Today, Jayan is best remembered as the first and best action star of Malayalam cinema, so far, besides his trademark colourful attire, risky stunts, machismo mannerisms and unique speaking style. He has rightly won immortality in the hearts of the Malayalam film fans as a martyr in his yearning to thrill and entertain them even by putting his life at stake. Madhu, a famous actor prominent in the 1960s, once stated in an interview: "Jayan will forever be young and alive. No one can ever visualise him as an old man."  A film titled Avatharam presently under production, is attempting to bring back his screen persona using advanced technologies. A documentary on Jayan's life and death Jayan - The Man behind the Legend is nearing completion for release in the near future.

Where is he most popular?
He has rightly won immortality in the hearts of the Malayalam film fans