I was curious to hear what the general consensus is regarding this.
I'll give a short list of the main candidates and their best results.
Paolo Bettini
Olympic Road Race: 2004
World Championship Road Race: 2006 and 2007
Liège-Bastogne-Liège: 2000 and 2002
Milan-San Remo: 2003
Giro di Lombardia: 2005 and 2006
Züri-Metzgete: 2001 and 2005
Clasica de San Sebastian: 2003
+ Top ten in the Ronde van Vlaanderen
Tom Boonen
World Championship Road Race: 2005
Ronde van Vlaanderen: 2005, 2006 and 2012 (shared record)
Paris-Roubaix: 2005, 2008, 2009 and 2012 (shared record)
Gent-Wevelgem: 2004, 2011 and 2012 (shared record)
E3 Harelbeke: 2004, 2005, 2006, 2007 and 2012 (record)
Kuurne-Brussel-Kuurne: 2007, 2009 and 2014 (record)
+ A second and third place at Milan-San Remo
Fabian Cancellara
Paris-Roubaix, 2006, 2010 and 2013
Milan-San Remo: 2008
Ronde van Vlaanderen: 2010, 2013 and 2014 (shared record)
E3 Harelbeke: 2010, 2011 and 2013
Strade Bianche: 2008, 2012 and 2016 (record)
+ Silver at the Olympic Road Race.
Philippe Gilbert
World Championship Road Race: 2012
Ronde van Vlaanderen: 2017 (first rider in 20 years to win both the Ronde and LBL in his career)
Giro di Lombardia: 2009 and 2010
Liège-Bastogne-Liège: 2011
Amstel Gold Race: 2010, 2011 and 2014
Flèche Wallonne: 2011
Clasica de San Sebastian: 2011
GP de Québec: 2011
Paris-Tours: 2008 and 2009
Omloop het Nieuwsblad: 2006 and 2008
Brabantse Pijl: 2011 and 2014
Strade Bianche: 2011
+ Two third places at Milan-San Remo and two third places at the Ronde van Vlaanderen. Only rider of this century to have been on the podium in 4 different Monuments.
Alejandro Valverde
Liège-Bastogne-Liège: 2006, 2008 and 2015
Flèche Wallonne: 2006, 2014, 2015 and 2016 (record)
Clasica de San Sebastian: 2008 and 2014
+ Two podiums at the Giro di Lombardia and six at the World Championships.
Oscar Freire
World Championship Road Race: 1999, 2001 and 2004 (shared record)
Milan-San Remo: 2004, 2007 and 2010
Gent-Wevelgem: 2008
Paris-Tours: 2010
Brabantse Pijl: 2005, 2006 and 2007
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Please keep the discussion to one-day races only.
I'll give a short list of the main candidates and their best results.
Paolo Bettini
Olympic Road Race: 2004
World Championship Road Race: 2006 and 2007
Liège-Bastogne-Liège: 2000 and 2002
Milan-San Remo: 2003
Giro di Lombardia: 2005 and 2006
Züri-Metzgete: 2001 and 2005
Clasica de San Sebastian: 2003
+ Top ten in the Ronde van Vlaanderen
Tom Boonen
World Championship Road Race: 2005
Ronde van Vlaanderen: 2005, 2006 and 2012 (shared record)
Paris-Roubaix: 2005, 2008, 2009 and 2012 (shared record)
Gent-Wevelgem: 2004, 2011 and 2012 (shared record)
E3 Harelbeke: 2004, 2005, 2006, 2007 and 2012 (record)
Kuurne-Brussel-Kuurne: 2007, 2009 and 2014 (record)
+ A second and third place at Milan-San Remo
Fabian Cancellara
Paris-Roubaix, 2006, 2010 and 2013
Milan-San Remo: 2008
Ronde van Vlaanderen: 2010, 2013 and 2014 (shared record)
E3 Harelbeke: 2010, 2011 and 2013
Strade Bianche: 2008, 2012 and 2016 (record)
+ Silver at the Olympic Road Race.
Philippe Gilbert
World Championship Road Race: 2012
Ronde van Vlaanderen: 2017 (first rider in 20 years to win both the Ronde and LBL in his career)
Giro di Lombardia: 2009 and 2010
Liège-Bastogne-Liège: 2011
Amstel Gold Race: 2010, 2011 and 2014
Flèche Wallonne: 2011
Clasica de San Sebastian: 2011
GP de Québec: 2011
Paris-Tours: 2008 and 2009
Omloop het Nieuwsblad: 2006 and 2008
Brabantse Pijl: 2011 and 2014
Strade Bianche: 2011
+ Two third places at Milan-San Remo and two third places at the Ronde van Vlaanderen. Only rider of this century to have been on the podium in 4 different Monuments.
Alejandro Valverde
Liège-Bastogne-Liège: 2006, 2008 and 2015
Flèche Wallonne: 2006, 2014, 2015 and 2016 (record)
Clasica de San Sebastian: 2008 and 2014
+ Two podiums at the Giro di Lombardia and six at the World Championships.
Oscar Freire
World Championship Road Race: 1999, 2001 and 2004 (shared record)
Milan-San Remo: 2004, 2007 and 2010
Gent-Wevelgem: 2008
Paris-Tours: 2010
Brabantse Pijl: 2005, 2006 and 2007
-----
Please keep the discussion to one-day races only.