There are so many to choose from, but amongst my favorite are those 3:
The Rest on the Flight into Egypt, an early work by Caravaggio from the collection of the great Doria-Pamphilj in Rome, so lyrical I could spend hours staring at it.
The Cheat with the Ace of Diamonds by the great 17th century painter from Lorraine, Georges de la Tour, one of the highlights of the Louvre.
Isle of the Dead by the Swiss Arnold Bocklin, which he painted several times between 1880 and 1886, my favorite version is the one that comes from the Metropolitan Museum in NYC, so moody and melancholy. (Ridley Scott made a visual reference to it in Alien: Covenant).