Why? (explain)

I suppose it's all based on opinion really. Someone might think it's totally unrealistic for Batman himself to exist... then again, I think it's a very fine line. I think it's highly possible for Batman to exist in our world. It seems less likely to me that he could keep his identity hidden in the "real" world, though.

I suppose I could come up with a list of characters I think are more likely to exist than those that are not...

Most likely:
Batman
Robin
Alfred
Batgirl
Catwoman
Commissioner Gordon
Riddler
Zsasz
KG Beast
Anarky
The Mad Hatter

Could be possible:
Scarecrow
Two-Face
Joker
Bane

Least possible:
Penguin
Mr. Freeze
Poison Ivy
Clayface
Killer Croc