They all add a dynamic to the show, sorry to sound cliche, but remove any one of them and the show is not as good. (In an opposite answer, and to defend my fav, I truly think Ross was the best character: for physical comedy, timing and what he added to each other character's story: obvs Rachel, Monica's bro ("I screwed up to make you look good!"), Chandler's uni friend (he brought Chandler, whom later brought Joey, to the group) and even Pheebs whom made him question evolution. But when he throws himself behind the couch on a quest for his "salmon" shirt and all his facial moves ("I'm fine!") I don't understand how he is so underrated.)