import re
string = "abc a bCd bC AbC BC BCD bcd ABC abc bcd bcd"
string = string.lower()
spisok = re.findall(r'\w+', string)
dic = {}
for i in spisok:
    if i in dic:
        dic[i] += 1
    else:
        dic[i] = 1
for keys, values in dic.items():
    print(keys, values)
print(dic)

abc 4
bcd 5
bc 2
a 1

>>> from collections import Counter
>>> s = "abc a bCd bC AbC BC BCD bcd ABC abc bcd bcd"
>>> Counter(s.lower().split(" ")).most_common()[0]
('bcd', 5)

>>> import operator
>>> d = {"a": 100, "b": 13, "c": 10000}
>>> d = sorted(d.items(), key=operator.itemgetter(1), reverse=True)
>>> d[0]
('c', 10000)

>>> d = {"a": 100, "b": 13, "c": 10000}
>>> 
>>> max(d.items(), key=lambda i: i[1])
('c', 10000)
>>>