[Foobarz]

So, is the reactivity series ranked in order of ionization energy from lowest to highest? (K, Na, Li. . .) or is it the opposite of the order of standard electrode potentials (Li, K, Sr. . .)

[Borek]

It is about potentials. They are partially related to ionization energy, but not only - other processes, like hydration of the cation, are also important.

[Foobarz]

So it goes Li, K, Sr?

[Borek]

That would be my approach.

Note that the "reactivity" in the context of "reactivity series" is not precisely defined, so in some cases order of the elements will depend on how you want to measure the "reactivity". It is not worth to treat the concept too seriously, especially at the end of the scale.