7. Janice

6. Rachel Green

5. Monica Geller

4. Chandler Bing

3. Phoebe Buffay
2. Ross Geller

1. And Joey Tribbiani

"Oh. My. GAWD!"
Janice, though never a ~main~ character, was one of the most memorable characters on the show. Her annoying laugh and her catchphrase, "Oh. My. God." made her stand out from other side characters and I couldn't leave her off this list.
Rachel was a good character, but when it came to the most memorable quotes, she only had a few. However, she did deliver one of the best lines of the show: "I got off the plane." She also had some funny lines that showed what type of character she was, like, "Oh I'm sorry, did my back hurt your knife?" But overall, her quotes were less memorable and less frequent.
Monica had some of the best lines of the series, though they weren't all as notable as others. The quote that stuck out most to me, was her roast of the other friends, including the "married a lesbian" and "threw a girl's wooden leg in a fire" bit.
Chandler was one of the funniest characters of the show. His advertising ideas such as, "Cheese. It's milk you chew!" were easily some of the funniest lines of the show. His "I'm not great at advice, can I interest you in a sarcastic comment?" is a perfect summary of most of what Chandler says.
Phoebe had some of the funniest, craziest lines of the series. From her made up names of "Regina Phalange" and "Princess Consuela Banana Hammock" and her notable songs such as "Smelly Cat," she has some of the most memorable quotes of the series.
Ross was one of the characters with the most consistent memorable lines. The way the lines were delivered not only made them immortal and to be never forgotten, but also made for some of the funniest scenes. From his most famous line of "We were on a break" to his funniest line, "PIVOT!", he easily takes second place for best quotes on the show.
Joey was one of the funniest characters, hands down. He had the most quotes of any one on the show that are still quoted by people today. His catchphrase, "How you doin'?" was funny from the start and never got old. He also had my favorite quote of the entire series: "Joey doesn't share food!"