I assume this is about who is the most hated and not who I personally hate. So the answer is Ross.
Personally though I think my least favorite was always Joey. He had his funny moments but for the most part his role was just straight up stupid and only got dumber as the seasons went on. His was the only character that truly became a caricature of its former self and his ending plotline was just so pathetic and became even moreso since the spin off was cancelled after like a second.
