Oh I didn't know about the chaingrab. Falco has a hard time approaching IC's which probably ends up into a chain throw = 1 stock. GaW "counters" Falco but that's all I suppose.Kirby and Pikachu are both worse than GAW, and probably Marth too. Pikachu in addition to being a fast small target, also chain grabs falco to 100%, that's almost as bad for falco as the IC's chain grab and pikachu is a better character than the IC's otherwise as well. GAW actually isn't amazing against falco, he counters him, but he's not the worst. I think it's definitely IC's or Pikachu.
I have to disagree on Marth though. Many people say he's one of Falco's worst matchups but that's not true. Laser camping is easy to pull of but Marth has more priority up close which causes trouble but Marth can be handled pretty well. Reflector is useful also.
Kirby......well not many moves can work on him in all situations. That floating is more trouble than most people think.