Python-сообщество

 def invert(t):
    return max(t.split(' '), key=lambda x: t.lower().count(x))

  
>>> import collections
>>> 
>>> def f(s):
...     counted = collections.Counter(map(str.lower, s.split()))
...     out = counted.most_common(1)[0][0]
...     return out
... 
>>> f('abc DEF ghi ABC hij')
'abc'
>>>

  
>>> def f(s):
...     dct = {}
...     for i in map(str.lower, s.split()):
...         dct[i] = dct.get(i, 0) + 1
...     out = max(dct.items(), key=lambda i: i[1])[0]
...     return out
... 
>>> f('abc DEF ghi ABC hij')
'abc'
>>>