What would be the easiest (in terms of effort and time) way to get a PhD that is somewhat legit (no diploma mill or fake university)?
The field or subject doesn't really matter.
First of all, funding is not an issue. Tuition is free, and we pay a stipend (something like $18k/year I think), provided that you are willing to spend ~15 hours/week as a TA for calculus.
That said, in practice, the question of "what's the weakest possible dissertation you can get away with, and still earn a PhD" doesn't arise much. We have a battery of qualifying exams, which are a significant obstacle. After that, depending on your advisor's personality, you could possibly skate by with a weak dissertation. Weaker students tend not to finish at all.
It's often said that a PhD is a "labor of love". And indeed, in the US, people don't generally seek out PhDs with ulterior motives. This is an interesting contrast to e.g. Germany where politicians are sometimes discovered to have plagiarized their dissertations. In the US no one would bother, a PhD wouldn't help you get elected in the first place.
So (at least in the context I'm familiar with) the answer is somewhat ambiguous, probably depends on your definition of "somewhat legit", and certainly would depend on random factors which couldn't be predicted in advance.
- The qualifying exams are likely to be easier than at top tier universities;
- The faculty will be more accustomed to weak students with little motivation. Not all students will be weak, and indeed many students, at whatever program, will be strong and motivated. However, weaker programs are more likely to take chances on applicants with ambiguous track records.
- That said, it's certainly possible that individual professors, at whatever institution, decide that they aren't willing to put up with any shit.
To get a legit PhD or DSc, you would need to become an expert in the field - spending time doing research, projects, a thesis, and possibly teaching. Due to this, the easiest subject to select would be one you are already know alot about, hopefully via an undergraduate or graduate degree. This helps with the entrance qualifications too.
Also, just because it is easiest does not mean it's best. The subject should be something you are passionate about. You should also look at your return on investment for the degree. If you aren't going to benefit from it and you just want to add doctor to your title, you might as well use a diploma mill or 'fake' university.
I looked into doing a PhD for computer/information/data science. The best option I found was a DSc from Dakota State University. It was relatively cheap, online, accredited, and related to my job. I decided not to do it because I felt the investment in time and money would not be worth it. It would be a minimal raise at my current job. If I switched companies, then I would need experience to go with the degree as it seems that companies value that more.
Like most things, what you get out of a degree is based on what you put into it. If you want a legit doctorate, it should not be easy.
I guess the point that I didn't bring up is that for any degree, the public opinion of the institution matters - community college vs state school vs private vs ivy league, etc. Personally, I don't like the group-think stereotyping that goes on in those comparisons.
If that's not good enough, buy a PhD from some random sham "school."
If that's not good enough, go work at a random, exclusive, graduate school department of your desired university as an employee until they let you in their program. (I saw someone do this at [one of: Stanford, Berkeley or CalTech] and now they're a big-shot at Cedars-Sinai.)
But why would anyone want to add "Dr." to their email signature? The only ones who do that, in my experience#, tend to be either narcissistic or deeply insecure. Folks with lengthy creds don't tend to rub colleagues' noses in it, and many societies at-large tend toward anti-intellectualism. (If you go to Harvard or Yale and other people aren't WASPs, then you say you go to Northeastern or NYU.)
# I worked at a department with a 2% acceptance rate that had multiple TED speakers, and also a grad student married a decabillionaire.
I self-published a book on ALS (the disease) research [0]. I wonder if this would be enough to be accepted as a PhD student.
Also worth noting that if you're in the UK (possibly other non-US countries too) you wouldn't have to go through the whole qualifying exam ringer which cuts down the effort slightly and the expected number of years until thesis submission to 3-4.