l= ['a', 'b', 'c']
d = {'1': ['a', 'd', 'e'], '2': ['b', 'c', 'f'], '3': ['b', 'a', 'e']}

high = -1
key = []
for k,v in d.items():
    occ = (len(l) + len(v)) - len(set(l + v))
    print((set(l+v)))
    if(occ >= high):
        if(occ == high):
            key.append(k)
        else:
            key = [k]
        high = occ
print(key)