I prefer mine own view, but your three-woman theory of the show is very workable.
Here’s a good breakdown of screentime and speaking time from the Friends:
https://yashuseth.blog/2017/12/29/data-analysis-lead-character-of-friends-data-science/
Phoebe is pretty consistently in last. What’s interesting is that Monica is dead last in screen time on her own without any other friends present.
My initial thought was Phoebe for the reasons stated, but after thinking about it for a while, it’s Ross. Yes, many of the major plot lines wouldn’t have worked as written, but they could have written other plot lines, and Ross is the least funny and least interesting character on Friends.
Thanks guys. The analogy occurred to me when I tried to explain why I always thought this was a show about the women. We want to see what happens to them, we don’t care as much what happens to the men. Monica is bossy like Moe, and everybody sort of pushes around Rachel. Phoebe is that 3rd wheel that keeps them apart and brings them back together as needed, and has her own offbeat style of humor. Allow me to feel proud of myself momentarily, this is so good I should write it up for Friends Monthly. There is such a publication right?
I voted for Joey, and he is my least favorite character, but it’s why he was my least favorite. He devolved.
Monica was the anchor character, which isn’t quite the same thing as being the main character, but she was like Alex on Taxi, or Andy on WKRP in Cincinatti. There’s a reason her apartment is the main set. She grounds the others.
But she still grew as a person. She grew the least, and anchor characters do (or, don’t), but she certainly didn’t devolve. Ross grew, Chandler grew, Phoebe grew. Rachel grew the most, but she was such a terrible person to begin with, that she still wasn’t all that nice when the show ended-- but she was better.
Joey, on the other hand, regressed. He wasn’t stupid at the beginning of the show. For most of the run, he thinks he’s suave, but he really isn’t. He’s successful with women, and that’s kind of a mystery, because he’s such a 2-dimensional jerk. Now, the Joey of the first half of the first season, I can see being successful with women-- he actually is suave, and he’s not obsessed with food. He has a distinct style of dress.
But he regresses until he is practically a child by the end of the show. I think it was one of the worst choices the writers made.
Phoebe, on the other hand, had a purpose. Because she was so quirky, pretty much anything she came up with was believable, so when the writers needed to handwave something in, give it to Phoebe. The gang needs a car for the weekend? Oh, Phoebe’s grandmother drives a cab, which she’s willing to lend them for the weekend. Want Monica to obsess over a cookie recipe? Have Phoebe announce that she knows the perfect one-- only she lost it.
Every season there were about a half dozen things that needed to be handwaved in, and Phoebe was the vehicle-- and that way, they made perfect sense. They wouldn’t have made perfect sense from any other character.
So, she’s not just a quirky character tacked on for extra comic opportunities. She’s there for a reason.
Joey, I think, is supposed to be there to provide contrast to unslick Ross and Chandler, but Joey is so unconvincing as a ladies’ man, it doesn’t work.
It actually was the other way around; Ursula came first. Lisa Kudrow began playing Ursula on Mad About You in 1993; the producers of Friends liked her in that role, and when they cast Kudrow as Phoebe in '94, they decided to make Phoebe and Ursula sisters.
Phoebe is my favorite character of the six, and even if she was less involved in the main plotlines, I would never have cut her out. 
Except Joey really came through in the clutch for Rachel when she was pregnant. I’m not sure I recall Phoebe being anything more than a quirky friend with an odd personal history that nobody knew much about except when it suddenly came up in conversation. It’s like she had a completely separate life from the rest of the “friends” and seamlessly floated in and out of the group.
It was good for the overall storyline for Phoebe to add novelty now and then. But mostly she seemed more of an outsider. Therefore, more expendable and replaceable as a character. Though I’m glad they kept her around.
I’m going to go with Gunther.
Ross is the least funny because he’s the ‘straight guy’ - but without him, Phoebe could never have won against evolution!
True, but I think that does support that he’s the most expendable. You can have different characters fill the straight guy role in different circumstances without having a dedicated straight guy. Like, in the Chandler/Joey duo, Chandler is more often the straight guy, and it works fine.
I always felt like Ross was supposed to be the “adult” of the group. They never made it super clear but I assumed he was the oldest. He was the first to get married (and married), the first to have a kid, he had the most stable career, etc.
And frankly, “The One Where No One’s Ready” is the best episode and it wouldn’t work with anyone else in the Ross role.
You could lift Phoebe out of Friends the same way you could lift Kramer right out of Seinfeld. You’d still have workable plots without them, but they wouldn’t be as good.
The writers sort of got around the question by having Chandler be the most disposable in the first few seasons, then Joey for awhile, then Ross for a season or two, then Phoebe. When you get right down to it, the only two “indispensable” characters were Monica and Rachel.
I choose—Eddie Haskell !
Ross was supposed to be the adult of the group. Unfortunately, David Schwimmer happened. Ross being the adult meant fewer crazy antics or interesting plotlines, so they started with giving him a monkey and got weirder from there. It was almost as bad as what happened to Joey…
I’ll go with Treeger.
Marcel.
This makes me think (sorry). It could get kinda meta if you’d like: maybe Phoebe was intentionally written as someone who is quirky, more introverted than extro, and not comfortable being the center of things. It’s natural then that some of Phoebe’s biggest fans share some of these traits. If the show wrote Phoebe to be more integral to the group’s relations with fingers deep in everybody’s pies then it wouldn’t be true to her established character and thus disappointing to her fans.
or something
i guess the Meta or ironic thing would be if Phoebe had been written as much more central to the show it might have ruined the quirky attributes that made folks like her in the first place.
I get the feeling that Phoebe’s ‘alternative’ character was written just to highlight the superiority of the other characters’ middle-of-the-road outlook. Ross is continually mocked for having any interest at all in academia/intellect; Phoebe demonstrates that alternative lifestyles are hypocritical and superficial. She’s not particularly principled with her vegetarianism or opposition to marriage, and never speaks up against the insidious homophobia running through so many scenes. So I’d certainly cut her out if we had to lose someone.
This poll is depressing. Friends was a terrible show; the only possible reason to watch it was Lisa Kudrow. She is one of a handful of actors I will ALWAYS watch, no matter what they’re in. If you haven’t seen The Opposite of Sex, Romy & Michelle’s High School Reunion, or The Comeback, you are missing out on one of the most gifted natural comic actors of all time.
I hate that I just voted Phoebe, because I was going with what I thought the masses would do, rather than my own preference. I loved Phoebe.
In my world, I would kick out Chandler who I always found really annoying and trying too hard to be funny. Which I realise would get me kicked out of all ‘Friends’ appreciation societies.