What element has the lowest first ionization energy?
If you follow the general trend on the periodic table, you see that ionization energy decreases down a period because as electrons are added to higher octets, the average distance of the electron from the nucleus increases and screening by inner electrons increases. This means the electrons are easier to remove because the nucleus does not hold them as strongly.
Ionization energy also decreases from right to left because atoms on the left side of the periodic table can get to a noble gas configuration more easily by losing electrons than by gaining electrons, so they are more willing to let electrons go.
Following the trend, it makes sense that the element with the lowest first ionization energy would be the lowest, leftmost element, Francium.