For me I think it was:
Rachel - Series 3. She had ambition in her new job and came across intelligent and willing during the Mark/Joanna phase in the office, whereas later on she became aloof, a bit of a "bimbo" and had relationships/love interests with her colleagues and clients (joshua/tag/gavin)
Monica - Season 6. She was head chef at the restaurant where she'd reached good success and respect. Her and chandler were great in their relationship too. Before this she was building up to her career and relationship and afterwards she became very shrill and overly bossy in everything.
Pheobe - season 4. Prior to this she was very sweet, hippy type, very loyal with a lovely innocent charm about her. She had the triplets for her brother then afterwards became a very hardened, colder, tough love type of character and often quite mean.
Ross - season 5 and 6. He became less serious, he was funny once his character became less uptight, the paste pants/the spray tan/the teeth, the "Elizabeth " storyline. He became funnier as the seasons went on.
Chandler - season 3. I loved him throughout but his best comedy was his dry humour and although he was great with Monica, his character in a comedy sense at work/with the group/dating was imo season 3. He was funny later on but he became soft and almost downtrodden by Monica.
Joey - season 3/4. When he was slightly dim but cute and before becoming really stupid (such as becoming a sex pest to Rachel and ross's nanny and talking on the phone a Teddy bear/the learning French episode.) When he was smart enough to read The Shining yet cute enough to put it in the freezer. Days of Our Lives/Kathy/Kate etc. He was capable of creating real feelings with women and was genuinely gutted when they didn't work out.
Obviously just my opinion 😆 Feel free to add yours 🙂