This chart indicates who held the position of longest reigning European monarch at any time in European history since 1492, the year of the unification of Spain and the discovery of the New World, with Wikipedia links provided for more information. Many of the towering figures of European royal history appear, as well as some lesser-known ones. Unsurprisingly, it is Louis XIV of France who held the status of senior monarch for the longest: the last fifty years of his 72-year reign.
Prior to the 1871 unification of Germany, I suppose it is possible at any point that there was a prince of a little principality somewhere who had been reigning longer than any of the more famous monarchs listed, but these rulers while often de facto independent were arguably not fully sovereign, being first subject to the Holy Roman Empire and then to the Germanic Confederation. The unifications of Italy and Germany dramatically increased the importance and visibility of the few small states that remained independent. Therefore prior to 1871 I have considered only rulers of kingdoms and empires, with sovereign princes, grand duchesses etc. eligible for inclusion only after that date (which is why Ivan the Terrible's accession is dated from 1547 when he proclaimed himself the first Tsar of all the Russias rather than 1533 when he became Grand Prince of Moscow). (If for the sake of consistency smaller states were excluded entirely, after the death of King Haakon VII of Norway in 1957 the senior monarch would have been Paul I of Greece (1947) until 1964, Frederik IX of Denmark (1947) until 1972, Gustaf VI Adolf of Sweden (1950) until 1973, Baudouin I of Belgium (1951) until 1993, and Queen Elizabeth II since 1993.)
In cases of monarchs (Augustus "the Strong" of Poland, Philip V of Spain, and Ferdinand IV/III/I of Naples & Sicily) who temporarily renounced or lost their thrones (though Ferdinand never lost Sicily) but then came back to reign for the rest of their lives, I have considered their first accession date to be valid for the purposes of seniority. Therefore if Simeon II of Bulgaria (1943) were restored today, as I fervently wish he would be, he could be considered "senior" to Queen Elizabeth II (1952).