What cancer is most likely to metastasize?

Leukemia: White cells by their nature invade tissues, so by definition, a leukemia (cancer of the white blood cells) has always metastasized. There are a few thousand known types of cancer; oat cell of the lung is essentially always metastatic at the time of presentation, while a basal cell carcninoma can eat from the face into the brain with almost no chance of metastasizing.