Most Influential = created a new subgenre,  or a publishing phenomenon that others tried to emulate, or created some other movement in the market (BOOKED TO DIE, for example), or books by mega-stars of the genre.  The range of settings and plots of the mystery/crime/thriller novel was greatly expanded during the last 50 years because of the novels on the list below:

(In chronological order by date of publication)

THE GODFATHER by Mario Puzo (1969)

THE BLESSING WAY by Tony Hillerman (1970)

THE HOT ROCK by Donald E. Westlake (1970)    

THE NEW CENTURIONS by Joseph Wambaugh (1970)

FADEOUT by Joseph Hansen (1970) 

THE FRIENDS OF EDDIE COYLE by George V. Higgins (1972) 

THE GODWULF MANUSCRIPT by Robert B. Parker (1973)   

THE SEVEN-PER-CENT SOLUTION by Nicholas Meyer (1974) 

WHERE ARE THE CHILDREN? by Mary Higgins Clark  (1975)

THE EAGLE HAS LANDED by Jack Higgins (1975)

EYE OF THE NEEDLE by Ken Follett (1976)

RAISE THE TITANIC! by Clive Cussler (1976)

COMA by Robin Cook (1977)

A MORBID TASTE OF BONES by Ellis Peters (1977) 

THE LAST GOOD KISS by James Crumley (1979) 

GORKY PARK by Martin Cruz Smith (1981) 

A IS FOR ALIBI by Sue Grafton (1982)

INDEMNITY ONLY by Sara Paretsky (1982)

LA BRAVA by Elmore Leonard (1983)  

THE HUNT FOR THE RED OCTOBER by Tom Clancy (1984)

PRESUMED INNOCENT by Scott Turow (1986)

THE FIRM by John Grisham (1991)

THE CAT WHO SAW RED by Lilian Jackson Braun (1986)  

THE BLACK DAHLIA by James Ellroy (1987)  

SILENCE OF THE LAMBS by Thomas Harris (1988) 

CATERING TO NOBODY by Diane Mott Davidson (1990)

THE DEVIL IN A BLUE DRESS by Walter Mosley (1990)  

POSTMORTEM by Patricia Cornwell (1990)

BOOKED TO DIE by John Dunning (1992)                                               

THE BLACK ECHO by Michael Connelly (1992)

ALONG CAME A SPIDER by James Patterson (1993)

ONE FOR THE MONEY by Janet Evanovich (1994)

KILLING FLOOR by Lee Child (1997) 

THE DA VINCI CODE by Dan Brown (2003)

THE GIRL WITH THE DRAGON TATTOO by Stieg Larsson (2008) 

GONE GIRL by Gillian Flynn (2012)