Most common letter in a list
There are a few problems with this; mainly that you only support the letters a, b, and c. It isn't necessarily a problem if those are the only letters you'll ever get, but what is a problem is that you're making identical variables, with identical behavior, for each letter. Then for every letter you want to add in the future, you have to repeat yourself. Anytime you see yourself with a pattern like this, it is screaming for you to use a dictionary, or situationally a list of lists.

Making that change, we'd end up with something like this

counter = {}
for letter in my_list:
    if letter not in counter:
        counter[letter] = 0
    counter[letter] += 1

largest = 0
best_letters = []
for letter, count in counter.items():
    if count &gt; largest:
        largest = count
        best_letter = [letter]
    elif count == largest:
        best_letter.append(letter)

print()
for letter in best_letters:
    print("Letter which repeats the most is: {}".format(letter.upper()))


Now we don't need the separate logic for each possible letter; we also don't need that hacky loop where you find the largest and then loop through.

We can make this better though, as you implied with collections.Counter. A Counter already does a lot of this work. For example, to get the count we just do

counter = Counter(my_list)


Then to get the best one, we just do

best_letter, largest = counter.most_common(1)[0]


This makes your total program look like this

best_letter, _ = Counter(my_list).most_common(1)[0]
print()
print("Letter which repeats the most is: {}".format(best_letter.upper()))


In general, it is always good to write functions to encapsulate behavior, but in this case you can probably just inline the use of the Counter.

Now if we want to get all of the letters that are most frequent, we can change it pretty easily. We'll do this instead:

most_common = counter.most_common(len(counter)) 
largest = most_common[0][1]

for letter, count in most_common:
    if count &lt; largest:
        break

    print("Letter which repeats the most is: {}".format(letter.upper())


That'll grab all of values in the order of most to least frequent, and then you loop through all of the ones that are most commonly seen. Note that there is no implicit sorting within the ones that share a frequency; if you want further constraints on that it'll take additional logic.