I don't know if I have favorite paintings, but I do have favorite painters.
I like anything by Botticelli. I think he was just one of the most talented painters of all time.
Actually, my profile pic is one of Boticelli's paintings.
If you're in a darker mood, though, I think anything by Albrecht Durher is amazing.
My favorite works of art have to be sculptures, though. All the ancient Greek and Roman reliefs and statues.
When the barbarians sacked Rome, they deliberately destroyed many of the statues by knocking their heads off. For some reason I think there is just something so mezmerizing about these headless gods and goddesses.