There’s something I’ve wondered for some time now. By general consensus, who is considered the most liberal U.S. Supreme Court member to-date, and who is the most conservative.
Thank you in advance to all who reply 
By what criteria? Arguably any Supreme Court Justice who was in favor of maintaining the institution of slavery is far more “conservative” than Scalia, Rehnquist, or any of the modern-day right-wing justices.
My own inchoate sense, which I can articulate further if anyone’s interested, would be that Arthur Goldberg is the most liberal Justice, jurisprudentially, and James Clark McReynolds probably the most conservative.
I should also say that I am thinking more in the range of the 20th and 21st Century for this.
Why? What’s your cut-off date? (Note that both my picks are from the 20th century.)
I choose this time period because as you pointed out, in the distant past there were things like slavery that made the political climate much different than today. Arbitrarily, I am choosing the time from Jan. 1, 1900 to today.
Well, my answers don’t change – Arthur Goldberg and James Clark McReynolds – but I will note that the title of your thread is inaccurate, as you’re not really asking about the most liberal and conservative justices “to date.” 
Why MacReynolds in particular as opposed to the other three Horsemen?
–Cliffy
Because it’s my understanding, particularly after having read Barry Cushman’s Rethinking the New Deal, that McReynolds’s jurisprudence, more than any other Horseman, was borne less from a nuanced and pragmatic approach to the doctrinal underpinnings of classical legal thought, and more from his personal ideological convictions.
As it says in his Wiki entry:
Plus, he started serving on the Court earlier than any of the other Horsemen.
I’ll bite. Especially given his short tenure on the Supreme Court, why Goldberg rather than Douglas or Brennan?
I remember, btw, reading the headline at the time about his leaving the Court to become ambassador to the U.N., but I was pretty young then, and didn’t think much of it either way; I didn’t realize at that age just how different in power and prestige the two posts are. From this vantage point, though, it’s pretty incredible that even a talker like Lyndon Johnson was able to talk someone off the Supreme Court in order to become U.N. Ambassador.
Two words: Ninth Amendment.
Damn, you’re right! He was almost as liberal as the Founding Fathers! 
OK, I’m being facetious here. Don’t mind me. If I have anything intelligent to say, I’ll come back later.
My thought upon reading the OP was “Brennan and the 4 Horsemen (4 way tie).” But I’ve been convinced by your arguments on McReynolds. And Goldberg is a great choice, if for no other reason than he laid the groundwork that many of Brennan’s most influential decisions were laid upon. (Goldberg was the Chuck Berry of liberal Supreme Court Justices.)
But it’s difficult to overstate William Brennan’s influence on American jurisprudence. He had influential votes that swung the judiciary to the left on abortion rights, civil rights, voting rights, free speech, death penalty, the Establishment Clause, and criminals’ rights. Off the top of my head, I believe he wrote the opinions in Texas v. Johnson and New York Times v. Sullivan (if I remember correctly – a big “if” – one of my law school professors said that Brennan’s decision in *NYT v. Sullivan * was the first case to consider a “chilling effect” on speech). He’s one of the most-influential Justices ever, and is practically the embodiment of a Justice who believes in a “living Constitution.”
So I think Goldberg was one of those stars that burns brightly, but burns out quickly. But Brennan gets my vote based on the sheer volume of liberal-osity (which is a totally cromulent word) in his long tenure.
I agree with everything you say, AQA.