So many, so little time! Here's a few I love:
Two connected by a theme
Uccello's
Hunt in the Forest
This is an astonishing painting in that it is the essence of an entirely new way of seeing that world...
Brueghal's
Hunters in the Snow
There is something that perfectly captures the essence of winter and of late mediaeval Dutch life here.
I love that period of art around the early C20th - probably the most fertile period of human creativity since the Renaissance. In particular: Kandinsky. All of his work from his earlier folky stuff to his later abstracts - here's an early thing:
Colourful Life
Paul Klee...
Ad Parnassum
I love Japanese print-making - Hokusai and Hiroshige and others. This is Hiroshige's
Bridge in the Rain - again, an absolutely revolutionary work that transformed western art too.
The very Japanese influenced Hundertwasser (who also trained as a print-maker there)... this is
Der grosse Weg
There are lots more!