The School of Athens by Raphael. It's my favorite because it's an uplifting ode to the unending human pursuit of knowledge, one of our noblest characteristics.
Other personal favorites:
Landscape with the Fall of Icarus often attributed to Pieter Bruegel the Elder. I love it because I just really like the story of Icarus, but also because of its unabashed reminder that the universe doesn't give a shit about you.
Napoleon Crossing the Alps, 1803 version currently at the Belvedere, Vienna, by Jacque-Louis David.
Human greatness, human ambition, human arrogance. Love it.