Maximum Occurrence Letter

[nilamo]

Using collections is probably cheating, but using a dict might be fine:

>>> def max_char(string):
...     chars = {string.count(ch): ch for ch in string}
...     most_used = max(chars.keys())
...     return chars[most_used]
...
>>> max_char("foo bar")
'o'
>>> max_char("spam eggs")
's'
>>> max_char("on a bright sunny day in august")
' '

The important part, is that you actually understand how it works, whichever one you turn in.