According to Wikipedia, the most recent common ancestor of all reigning European monarchs is now Louis IX, Landgrave of Hesse-Darmstadt. I checked to see how they trace descent from him, and this is how it goes:
United Kingdom
Spain
Netherlands
Belgium
Sweden
Norway
Denmark
Luxembourg
Liechtenstein
Monaco
United Kingdom
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->King Frederick William III of Prussia->Tsarina Alexandra Feodorovna (née Princess Charlotte of Prussia)->Grand Duke Konstantin Nikolayevich of Russia->Olga Constantinovna of Russia, Queen of Greece->Prince Andrew of Greece and Denmark->Prince Philip, Duke of Edinburgh->King Charles III
- Louis IX, Landgrave of Hesse-Darmstadt->Grand Duke Louis I of Hesse->Grand Duke Louis II of Hesse->Prince Alexander of Hesse and by Rhine->Prince Louis of Battenberg, 1st Marquess of Milford Haven->Princess Alice of Battenberg, Princess Andrew of Greece and Denmark->Prince Philip, Duke of Edinburgh->King Charles III
- Louis IX, Landgrave of Hesse-Darmstadt->Grand Duke Louis I of Hesse->Grand Duke Louis II of Hesse->Prince Charles of Hesse and by Rhine->Grand Duke Louis IV of Hesse->Princess Victoria of Hesse and by Rhine, Marchioness of Milford Haven->Princess Alice of Battenberg, Princess Andrew of Greece and Denmark->Prince Philip, Duke of Edinburgh->King Charles III
Spain
- Louis IX, Landgrave of Hesse-Darmstadt->Grand Duke Louis I of Hesse->Grand Duke Louis II of Hesse->Prince Alexander of Hesse and by Rhine->Prince Henry of Battenberg->Victoria Eugenie of Battenberg, Queen of Spain->Infante Juan, Count of Barcelona->King Juan Carlos I->King Felipe VI
- Louis IX, Landgrave of Hesse-Darmstadt->Amalie of Hesse-Darmstadt, Hereditary Princess of Baden->Wilhelmine of Baden, Grand Duchess of Hesse and by Rhine->Prince Alexander of Hesse and by Rhine->Prince Henry of Battenberg->Victoria Eugenie of Battenberg, Queen of Spain->Infante Juan, Count of Barcelona->King Juan Carlos I->King Felipe VI
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->King Frederick William III of Prussia->Tsarina Alexandra Feodorovna (née Princess Charlotte of Prussia)->Grand Duke Konstantin Nikolayevich of Russia->Olga Constantinovna of Russia, Queen of Greece->King Constantine I of Greece->King Paul of Greece->Sofía of Greece and Denmark, Queen of Spain->King Felipe VI
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->King Frederick William III of Prussia->William I, German Emperor->Frederick III, German Emperor->Sophia of Prussia, Queen of Greece->King Paul of Greece->Sofía of Greece and Denmark, Queen of Spain->King Felipe VI
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->King Frederick William III of Prussia->William I, German Emperor->Frederick III, German Emperor->Wilhelm II, German Emperor->Victoria Louise of Prussia, Duchess consort of Brunswick->Frederica of Hanover, Queen of Greece->Sofía of Greece and Denmark, Queen of Spain->King Felipe VI
Netherlands
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->Wilhelmine of Prussia, Queen of the Netherlands->King William II->King William II->Queen Wilhelmina->Queen Juliana->Queen Beatrix->King Willem-Alexander
Belgium
- Louis IX, Landgrave of Hesse-Darmstadt->Amalie of Hesse-Darmstadt, Hereditary Princess of Baden->Grand Duke Charles of Baden->Josephine of Baden, Princess of Hohenzollern-Sigmaringen->Marie of Hohenzollern-Sigmaringen, Countess of Flanders->King Albert I->King Leopold III->King Albert II->King Philippe
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->Wilhelmine of Prussia, Queen of the Netherlands->Prince Frederick of the Netherlands->Louise of the Netherlands, Queen of Sweden and Norway->Louise of Sweden, Queen of Denmark->Princess Ingeborg of Denmark, Duchess of Västergötland->Astrid of Sweden, Queen of the Belgians->King Albert II->King Philippe
Sweden
- Louis IX, Landgrave of Hesse-Darmstadt->Amalie of Hesse-Darmstadt, Hereditary Princess of Baden->Frederica of Baden, Queen of Sweden->Sophie of Sweden, Grand Duchess of Baden->Grand Duke Frederick I of Baden->Victoria of Baden, Queen of Sweden->King Gustaf VI Adolf->Prince Gustaf Adolf, Duke of Västerbotten->King Carl XVI Gustaf
Norway
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->Wilhelmine of Prussia, Queen of the Netherlands->Prince Frederick of the Netherlands->Louise of the Netherlands, Queen of Sweden and Norway->Louise of Sweden, Queen of Denmark->King Haakon VII (né Prince Carl of Denmark)->King Olav V->King Harald V
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->Wilhelmine of Prussia, Queen of the Netherlands->Prince Frederick of the Netherlands->Louise of the Netherlands, Queen of Sweden and Norway->Louise of Sweden, Queen of Denmark->Princess Ingeborg of Denmark, Duchess of Västergötland->Märtha of Sweden, Crown Princess of Norway->King Harald V
Denmark
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->Wilhelmine of Prussia, Queen of the Netherlands->Prince Frederick of the Netherlands->Louise of the Netherlands, Queen of Sweden and Norway->Louise of Sweden, Queen of Denmark->King Christian X->King Frederick IX->Queen Margrethe II
- Louis IX, Landgrave of Hesse-Darmstadt->Amalie of Hesse-Darmstadt, Hereditary Princess of Baden->Frederica of Baden, Queen of Sweden->Sophie of Sweden, Grand Duchess of Baden->Grand Duke Frederick I of Baden->Victoria of Baden, Queen of Sweden->King Gustaf VI Adolf->Ingrid of Sweden, Queen of Denmark->Queen Margrethe II
Luxembourg
- Louis IX, Landgrave of Hesse-Darmstadt->Caroline of Hesse-Darmstadt, Landgravine consort of Hesse-Homburg->Princess Amalie of Hesse-Homburg, Hereditary Princess of Anhalt-Dessau->Prince Frederick Augustus of Anhalt-Dessau->Adelheid-Marie of Anhalt-Dessau, Grand Duchess of Luxembourg->Grand Duke William IV->Grand Duchess Charlotte->Grand Duke Jean->Grand Duke Henri
- Louis IX, Landgrave of Hesse-Darmstadt->Amalie of Hesse-Darmstadt, Hereditary Princess of Baden->Grand Duke Charles of Baden->Josephine of Baden, Princess of Hohenzollern-Sigmaringen->Marie of Hohenzollern-Sigmaringen, Countess of Flanders->King Albert I of Belgium->King Leopold III of Belgium->Josephine-Charlotte of Belgium, Grand Duchess of Luxembourg->Grand Duke Henri
- Louis IX, Landgrave of Hesse-Darmstadt->Frederica Louisa of Hesse-Darmstadt, Queen of Prussia->Wilhelmine of Prussia, Queen of the Netherlands->Prince Frederick of the Netherlands->Louise of the Netherlands, Queen of Sweden and Norway->Louise of Sweden, Queen of Denmark->Princess Ingeborg of Denmark, Duchess of Västergötland->Astrid of Sweden, Queen of the Belgians->Josephine-Charlotte of Belgium, Grand Duchess of Luxembourg->Grand Duke Henri
Liechtenstein
- Louis IX, Landgrave of Hesse-Darmstadt->Amalie of Hesse-Darmstadt, Hereditary Princess of Baden->Caroline of Baden, Queen of Bavaria->Sophie of Bavaria, Archduchess Franz Karl of Austria->Archduke Karl Ludwig of Austria->Elisabeth Amalie of Austria, Princess Elisabeth of Liechtenstein->Franz Joseph II, Prince of Liechtenstein->Hans-Adam II, Prince of Liechtenstein
Monaco
- Louis IX, Landgrave of Hesse-Darmstadt->Amalie of Hesse-Darmstadt, Hereditary Princess of Baden->Grand Duke Charles of Baden->Marie Amelie of Baden, Duchess of Hamilton and Brandon->Lady Mary Victoria Douglas-Hamilton, Hereditary Princess of Monaco->Louis II, Prince of Monaco->Princess Charlotte, Duchess of Valentinois->Rainier III, Prince of Monaco->Albert II, Prince of Monaco