Velasquez - Las Meninas - I was in awe of this at El Prado
Saturn devouring one of his children - Goya room - El Prado
Guernica - Picasso - I was lucky to see this at Reina Sofia
El Greco hall in El Prado
Rothko room - Tate Modern
Anything by Caravaggio
Crows in wheat field - Van Gogh exhibition in Basel Kunstmuseum

There are so many. I love art.