Which is the most reactive metal: Caesium or Francium?
The reactivity series consists of elements whose properties are FULLY known and understood. It is on the basis of whether an element can displace another from its salt solution (this is what you've been told at grade 10 Indian schooling level I guess?).
Francium, on the other hand is a radioactive element, with a half life of only 22 minutes for its most stable isotope
(Francium-223). Therefore, we can only make predictions, and not observations, about the nature of its salts and whether they would displace other elements from their salt solution or not.