Unpopular opinion: Phoebe is the best friend.
I see that Phoebe gets a lot of hate in this board. So maybe I'm alone here - but I find her to be the funniest and most interesting character. People call her out for being selfish but ALL of the Friends were selfish in different ways. At least she was interesting and different.
I also find Lisa Kudrow to be the strongest actor in the series. No hate to the others, because I honestly adore all of them in their own way. When watching the series and through all the re-watches of the series - I find that I look forward to the Phoebe story lines the most.
I've read all the arguments about why she is a "bad" friend, and I am not disagreeing that she acted in self-interest some of the time, or that the other friends tip toe'd around to please her - all true. STILL, she is the most human, funniest, and realest of the Friends to me. Pretty much all of the other Friends (except maybe Joey?) had fairly pampered lives financially. They all had their problems (Monica's weight struggles, Chandler's issues with his parents, Ross's self confidence issues, Rachel's princess complex.) but Phoebe grew up in poverty and had a truly tragic and hard life.
She was homeless and had to deal with the suicide of her mother and zero *beep* given by her selfish twin sister. She lived in a car and got hepatitis from a pimp that spit in her mouth! Having grown up poor myself, she was the only one that I could relate to in a real way. While I haven't been lucky enough to contract Hepatitis from pimp spit, I relate to the fact that Phoebe had her childhood taken away from her. As an adult, she just wants to reclaim the time she lost. Most of the time she is pushing people out of their comfort zones. Some see that as a selfish, I see it as her trying to show the other friends - Don't take life so seriously!
Basically, what Phoebe says during her pregnancy rage sums up how her problems compare to the problems of her friends.
Phoebe: Good. Y’know—no-no, okay, it’s-it feels like everything’s been about me lately, so what’s happening with you?
Rachel: Oh, well, actually we were just talking about me not going to Ross’s wedding.
Phoebe: Oh!
Rachel: It just might be too hard, given the history and all that…
Phoebe: Wow! This reminds me of the time when I was umm, living on the street and this guy offered to buy me food if I slept with him.
Rachel: Well, h-how is this like that?
Phoebe: Well, let’s see, it’s not. Really, like that. Because, you see that was an actual problem, and uh, yours is just like y’know a bunch of y’know high school crap that nobody really gives y’know…
Rachel: (starting to cry) I’m-I’m sorry, I just thought that…
Phoebe: Alrighty, here come the water works. (Rachel starts crying harder.)
TLDR: I love all the Friends - but Phoebe is the most human to me.