max(L,key=len) will give you the object with the longest length ([1,2,3] in your example) -- To actually get the length (if that's all you care about), you can do len(max(L,key=len)) which is a bit ugly -- I'd break it up onto 2 lines. Or you can use the version supplied by ecatamur.
All of these answers have loops -- in my case, the loops are implicit which usually means they'll be executed in optimized native machine code. If you think about it, How could you know which element is the longest without looking at each one?
Finally, note that key=function isn't a feature that is specific to max. A lot of the python builtins (max,min,sorted,itertools.groupby,...) use this particular keyword argument. It's definitely worth investing a little time to understand how it works and what it typically does.
