One particular reason why Creon can be seen as the most tragic of characters is because he is condemned to live.  Antigone dies in the pure knowledge and understanding that her path and her understanding is absolute.  Creon, on the other hand, is condemned to live, having to watch the brutal reality of his actions realized.  He is forced to watch his son die, his wife suffer, as well as his own sense of understanding about the world and his view of justice shrink in absolute terror.  It is Creon who undergoes the most amount of transformation and character evolution and, like Oedipus, gains understanding and wisdom only after having undergone immense suffering.

From my notes:

Tragic heroes are so much the highest points in their human landscape that they seem the inevitable conductors of the power about them, great trees more likely to be struck by lightning than a clump of grass. Conductors may of course be instruments as well as victims of the divine lightning.  (Northrop Frye)

According to this definition, Creon, as king, is the "highest point" of the human landscape and the greatest "conductor" of divine lightning.  All the tragic suffering is conducted by him toward others.  He suffers first and makes others suffer by extension.  Haemon and Eurydice are the lower points of the human landscape, the "clumps of grass," who are also struck down by the strike.

Death is also a deciding factor.  Although he doesn't die like Antigone, Creon suffers like Oedipus at the end of Oedipus the King.  His wife, son, and would-be daughter-in-law die.  It's a tragic cause and effect: hubris leads to bad law; hubris leads to stubborn rebellion of bad law; hubris leads to stubborn punishment of rebellion; hubris leads to hasty suicide.  Creon is left to clean up the pieces: his family's deaths, his subjects' rebellion, his cursed, lonely rule.

Really, the play involves two lightning strikes, two tragic heroes who present two extreme cases of hubris in the exercise of and reaction to law and power.  Sophocles, as much as he wants to be objective, sides with Antigone, I think.  He gives her the moral high ground, as she upholds gods' law above man's.