This can be run against a Slack export. It will count the reactions used and display them in an ordered list. Written for readability not speed or efficiency. No guarantees that this isn’t terribly broken. Enjoy and use responsibly!

import json
import glob
import collections

# collect messages
messages = []
for filename in glob.glob('*/*.json'):
    with open(filename) as f:
        messages += json.load(f)

# extract reactions
reactions = []
for message in messages:
    if "reactions" in message:
        reactions += message["reactions"]

# count reactions
reaction_counter = collections.Counter()
for reaction in reactions:
    reaction_counter.update({reaction["name"]: reaction["count"]})

# done, print them
print(reaction_counter.most_common())