Just my opinion...
1960 - Anthony Perkins in Psycho
1961 - Paul Newman in The Hustler
1962 - Peter O'Toole in Lawrence of Arabia
1963 - Marcello Mastroianni in 8½
1964 - Peter Sellers in Dr.Strangalove
1965 - Lee Van Cleef in For a Few Dollars More
1966 - Eli Wallach in The Good,the Bad and the Ugly
1967 - Paul Newman in Cool Hand Luke
1968 - Richard Burton in Who's Afraid of Wirginia Woolf?
1969 - Jon Voight in Midnight Cowboy
1970 - George C.Scott in Patton
1971 - Malcolm McDowell in A Clockwork Orange
1972 - Al Pacino in The Godfather
1973 - James Coburn in Pat Garrett and Billy the Kid
1974 - Gene Hackman in The Conversation
1975 - Jack Nicholson in One Flew Over the Cuckoo's Nest
1976 - Robert De Niro in Taxi Driver
1977 - John Travolta in Saturday Night Fever
1978 - Robert De Niro in The Deer Hunter
1979 - Peter Sellers in Being There
1980 - Robert De Niro in Raging Bull
1981 - William Hurt in Body Heat
1982 - Ben Kingsley in Gandhi
1983 - Al Pacino in Scarface
1984 - F.Murray Abraham in Amadeus
1985 - Eric Stoltz in Mask
1986 - Gary Oldman in Sid and Nancy
1987 - Max Von Sydow in Pelle the Conquerer
1988 - Jeremy Irons in Dead Ringers
1989 - Tom Cruise in Born on the Fourth of July
1990 - Robin Williams in Awakenings
1991 - Nick Nolte in Cape Fear
1992 - Harvey Keitel in Bad Liutenant
1993 - Michael Douglas in Fallung Down
1994 - Tom Hanks in Forrest Gump
1995 - Kevin Spacey in The Usual Suspects
1996 - William H.Macy in Fargo
1997 - Nick Nolte in The Affliction
1998 - Edward Norton in American History X
1999 - Kevin Spacey in American Beauty
2000 - Christian Bale in American Psycho
2001 - Russell Crowe in A Beautiful Mind
2002 - Ray Liotta in Narc
2003 - Min-Sik Choi in Oldboy
2004 - Kevin Bacon in The Woodsman
2005 - Heath Ledger in Brokeback Mountain
2006 - Leonardo Di Caprio in The Departed
2007 - Daniel Day Lewis in There Will Be Blood
2008 - Tom Hardy in Bronson
2009 - Sam Rockwell in Moon
2010 - Jeff Bridges in True Grit
2011 - Ryan Gosling in Drive
2012 - Joaqim Phoenix in The Master
2013 - Leonardo Di Caprio in The Wolf From Wall Street
2014 - Jake Gyllenhaal in Nightcrawler
2015 - Leonardo Di Caprio in The Revenant
Some years are really tough.1989 it could be Cruise,Lewis and Willaims. All three are equally good performances. 1990's were hardest. Many great performances. And for me booth Hopkins in Silence and Day Lewis in Gangs are clearly supporting roles. While Spacey in The Usual Suspects is leading by any standards.
1960 - Anthony Perkins in Psycho
1961 - Paul Newman in The Hustler
1962 - Peter O'Toole in Lawrence of Arabia
1963 - Marcello Mastroianni in 8½
1964 - Peter Sellers in Dr.Strangalove
1965 - Lee Van Cleef in For a Few Dollars More
1966 - Eli Wallach in The Good,the Bad and the Ugly
1967 - Paul Newman in Cool Hand Luke
1968 - Richard Burton in Who's Afraid of Wirginia Woolf?
1969 - Jon Voight in Midnight Cowboy
1970 - George C.Scott in Patton
1971 - Malcolm McDowell in A Clockwork Orange
1972 - Al Pacino in The Godfather
1973 - James Coburn in Pat Garrett and Billy the Kid
1974 - Gene Hackman in The Conversation
1975 - Jack Nicholson in One Flew Over the Cuckoo's Nest
1976 - Robert De Niro in Taxi Driver
1977 - John Travolta in Saturday Night Fever
1978 - Robert De Niro in The Deer Hunter
1979 - Peter Sellers in Being There
1980 - Robert De Niro in Raging Bull
1981 - William Hurt in Body Heat
1982 - Ben Kingsley in Gandhi
1983 - Al Pacino in Scarface
1984 - F.Murray Abraham in Amadeus
1985 - Eric Stoltz in Mask
1986 - Gary Oldman in Sid and Nancy
1987 - Max Von Sydow in Pelle the Conquerer
1988 - Jeremy Irons in Dead Ringers
1989 - Tom Cruise in Born on the Fourth of July
1990 - Robin Williams in Awakenings
1991 - Nick Nolte in Cape Fear
1992 - Harvey Keitel in Bad Liutenant
1993 - Michael Douglas in Fallung Down
1994 - Tom Hanks in Forrest Gump
1995 - Kevin Spacey in The Usual Suspects
1996 - William H.Macy in Fargo
1997 - Nick Nolte in The Affliction
1998 - Edward Norton in American History X
1999 - Kevin Spacey in American Beauty
2000 - Christian Bale in American Psycho
2001 - Russell Crowe in A Beautiful Mind
2002 - Ray Liotta in Narc
2003 - Min-Sik Choi in Oldboy
2004 - Kevin Bacon in The Woodsman
2005 - Heath Ledger in Brokeback Mountain
2006 - Leonardo Di Caprio in The Departed
2007 - Daniel Day Lewis in There Will Be Blood
2008 - Tom Hardy in Bronson
2009 - Sam Rockwell in Moon
2010 - Jeff Bridges in True Grit
2011 - Ryan Gosling in Drive
2012 - Joaqim Phoenix in The Master
2013 - Leonardo Di Caprio in The Wolf From Wall Street
2014 - Jake Gyllenhaal in Nightcrawler
2015 - Leonardo Di Caprio in The Revenant
Some years are really tough.1989 it could be Cruise,Lewis and Willaims. All three are equally good performances. 1990's were hardest. Many great performances. And for me booth Hopkins in Silence and Day Lewis in Gangs are clearly supporting roles. While Spacey in The Usual Suspects is leading by any standards.