All of that depends on the uses your intend for them and personal preference.
The KRK has the best build and folds flat. The other two offer a little more bass, but the AKG is a poor choice for portable use. The CAL! is light and comfortable, but lightly built. Only the CAL! comes with a cord suitable for portable use out of the box. Such a cord for the KRK will cost you another $15 to $20 shipped.
I suggest that you look up each in the buying guides that are linked in the opening post of this thread. Longer reviews of each are in turned linked from the buying guides.
Well they won't be for portable use I don't really care about that. It sucks that I can't just listen to them to see what I like more. I don't have any "good" headphones to compare to see what I really like. I know it is between the AKGs and KRK 6400. I would like something would good bass, but nothing that might overwhelm everything else. I have no idea if I would like something that is flat sounding or not. I read that the KRKs are suppose to be more accurate to how the song is suppose to be, but I wonder if it will make "low quality" songs sound worse. It seems that KRK might be a good headset for a person who has a diverse music taste though. The big thing the AKGs have going for them for me right now is price as they are only $62 on Amazon atm. Anyways thanks for the help.
Both the KRK and AKG are well respected studio quality cans. The one you pick may or may not be perfectly suited to your taste, but you will have a quality headphone in this general price range.