>>> animals = {'a': ['aardvark'], 'b': ['baboon'], 'c': ['coati'], 'd'
... : ['donkey',['lion', 'tiger', 'cat'], 'dog', 'dingo']}

>>> aDict = animals

>>>
>>> max(aDict.items(), key=lambda x: len(x[1]))[0]
'd'

>>> max((len(v), k) for k, v in aDict.items())[1]
'd'

>>> aDict = {'c': ['coati'], 'a': ['donkey',['lion', 'tiger', 'cat'],
... 'dog', 'dingo'], 'x': ['aardvark'], 'b': ['baboon']}
... # mix them up so the highest letter isnt the longest list
...
...

>>> max(aDict.items(), key=lambda x: len(x[1]))[0]
'a'

>>> max((len(v), k) for k, v in aDict.items())[1]
'a'

>>>