d = {'previous_f_key': {'1g': ['33725.7', '-70.29'],
                               '2g': ['35613.3', '108.83'],
                               '3g': ['32080.9', '-69.86']},
            'f_key': {'1g': ['8880.8', '-66.99'],
                      '2g': ['6942.6', '114.79'],
                      '3g': ['12300.3', '-70.34']}}
 
def weird_max(d):
    vals = []
    for item in d.itervalues():
        if isinstance(item, dict):
            vals.append(weird_max(item))
        else: # should be a list
            vals.extend(item)
    return max(vals, key=float)
 
print weird_max(d)

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