Pff. Rachel was by far the least annoying character, and probably not coincidentally also the one who grew up and developed the most during the course of the show, followed by Chandler. While she could still be superficial and vain, she was a hell of a lot more mature than Ross by the end.
Maturity has little to do with whether or not someone is annoying.
Aniston moaned her character a better job so she could wear more glamourous clothes on the show.
What did said job entail ? Who knows ? She has a standard over-designed TV career-woman office that's completely impractical for actually doing any work, designed by someone who has clearly never worked in one and filled with so much "stuff" that there's no room for that most essential modern office tool - a computer. Ralph Lauren runs on paper memos and handwritten documents, does it ?
The job that nobody can quite explain might have been funny if they hadn't already done that with Chandler.
Well, whatever she does for a living she must have been good at it since Louis Vitton were willing to move her to Paris, pay for her daycare and pay for Ross to visit his daughter whenever he wanted. Not only that, but her previous employers were even willing to take her back when she decided not to go.
That's why she's annoying. At least there was some semblance of reality to the others.