On tableau public, Skyler Johnson has created a handy data set that calculates the number of times each spell was used in all seven Harry Potter novels! It’s extremely handy and also very pretty.
The chart itself can be sorted by “Most Occurrences” and “Order of Appearance.” It’s important to note that none of the non-verbal spells were included in this data set, only spells that were spoken aloud. Perhaps not surprisingly, the spells that we see used most often are commonly used by Harry and his friends as means of non-violent resistance; Expecto Patronum; Expelliarmus; Stupefy. The Killing Curse appears sixth down on the list, which is fascinating–it is technically speaking the “most evil” of all the dark magic spells, and it is the most often used.
The rest of the chart can be found on tableau public, where you can fiddle around with the data and check all the citations!