What do pundits say?
In the explanatory dictionary of S. I. Ozhegov, the meaning of the word “last” is defined as “final in a series (numerical, temporary)”, “new”, “final”, as well as “bad”, “worst”. “Extreme” is “far”, “located on the edge”, “ultimate”, “strong”, “exceptional”. Obviously, both concepts are ambiguous and among them it is rather difficult to choose the correct option. But, if we are guided only by formal norms, then it is more correct to ask: “Who is the last in line?”. However, not everything is so clear. Why are punctuation the same in different languages.
What do ordinary people think?
In the human mind, the word “last” has a certain negative connotation due to the same Ozhegov's dictionary. Nobody wants to offend a stranger by calling him bad or worse. But, on the other hand, the concept of “extreme” also has a dark side. For example, you can make someone extreme by accusing him of a misdemeanor or transgression.
Now, think about what word would be more correct in the queue at the ATM.
A special case with people who have chosen dangerous professions: pilots, rescuers, firefighters, stuntmen. It is simply not customary for them to say “last day”, “last task”, “last flight”, so as not to accidentally bring trouble on themselves. Here, the “extreme” option is preferable. But, even despite the violation of the norms of the Russian language, such a replacement is quite acceptable and is used everywhere in colloquial speech.
So, in the context of the queue, it is correct to ask: “Who is the last one?”, But in reality, you can speak as you are used to or as your intuition suggests at the moment. In any case, you need to be tolerant and not correct people by demonstrating your awareness, but give them the opportunity to remain within their own picture of the world. Foreigners told what Russian speech is like.