I went to Spain as a teenager and saw Guernica in person. It was the first painting to ever really have an effect on me. It's stunning. A perfect example of how art can transmit a message between people across time and space, I just knew that I was feeling how Picasso wanted me to feel. If you have the chance to see this painting you should, no website can do it justice (although this is a very nice try).
That early, before the tour groups have made it that far into the museum, you can actually get the entire space to yourself for >30 minutes and it's never left me not weeping.