Function takes a dictionary with list as value, returns the key of the biggest list I created a function that takes a dictionary as parameter. This dictionary is expected to have lists as value pair. (i.e. {1: [1, 2, 3], 'a': [4, 'b', 5, 'c']} )
This function has to return the key associated to the biggest list in the dictionary, or None if the dictionary is empty. (considering the above example, it would return 'a' )
I came up with this code:
def biggest(aDict):
    return None if not aDict else [x for x in aDict if len(aDict[x]) == max(len(x) for x in aDict.values())][0]

Can I have some review on this and any ideas on how to further simplify without using external libraries?
Your code is difficult to read:

The line is too long (according to the Python style guide all lines should be limited to 79 characters).
The same iterator variable x is used for the outer iteration and also inside the max() function.

It is also inefficient because the maximal list length is determined in each iteration.
Using the key and default argument of the max() function the same can be concisely and efficiently be implemented as
def biggest(aDict):
    return max(aDict, key=lambda k: len(aDict[k]), default=None)

More suggestions:

Have a look at the Python naming conventions (snake_case, not camelCase).
Choose a more descriptive function name.
Use docstrings to describe the function.