you can use numpy:
import numpy as np
list1 = [1,2,4,5,7,6,5]
list2 = [1,4,5,6]
list3 = [2,5,4,6,7,8,5,3]
lists=[list1, list2, list3]
listsnames=["list1", "list2", "list3"]
most= listsnames[np.argmax([len(l) for l in lists])]
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