What should be the order of the authors in a paper? Should the first author be the one that worked the most? And then by the amount of work order (descending), but the last one the most-senior researcher?

Are there other ways? Which ones?

Does it vary from field to field? I've heard some areas or even some conferences always ask for alphabetical order. But then there are evaluation committees that count for how many first-author or last-author publications does a researcher have. Can someone provide exhaustive insight into how this is done across many fields?
Ask a hundred academics and get one hundred differing answers. Disciplines do differ. 


Some base in on who did the most work.
Some base it on who developed the idea.
Some base it on a hierarchy  (my lab me first).
Some do alphabetical.
Some do "equal author contributions."
Some horse-trade, I get first on this as you got first on last. Many types of horse-trade exist and some move outside the domain of the research.
Some do it based on need. I need to be first to keep my job but you do not. I need it for tenure and you are tenured.
Some do it based on who wrote most of the paper.
Some do it on random numbers or similar.
Some decide in advance before the process starts.
Some give it to the person who made the big breakthrough.
Some give it to the organizer who pushed everyone and harassed everyone to get it done.
Some people just do not care so give it to the person who asks first.
Some give it to the person who got the funding.


My process is that you essentially know by the end of a research process and writing of a paper who should be the lead author and then the horse-trading begins. I have fun with it and I really dislike teams who spend more time on discussing this than the research.