*sigh*
Please read this:
http://smashboards.com/showthread.php?t=108512
I think you mean "Who is most susceptible to shine combos / Up-Down combos?".
To which I would answer "Any fast faller.". Fox is especially vulnerable to shine combos because he falls faster than other characters, and when being juggled, doesn't recover as easily as more floaty characters do.
Captain Falcon is also fairly easy to juggle mercilessly, as are other Falcos.
Beyond these 3, the rule of thumb becomes: The more floaty the character, the harder they are to continuously combo. So characters like Link, Pikachu and Mario, who aren't fast fallers, but also aren't super floaty, are pretty consistently comboable. On the other hand, characters like Jigglypuff, Peach, Zelda or Luigi, are very floaty, making them less chain-comboable.
Random side note. Comboable is my new word for the day.