I need to maintain my log-in credential for throughout the app so, I have 3 options to maintain my log-in credentials...
1 key chain
2 NSUserDefaults
3 plist
>
I'll suggest keychain for storing the login credentials due to the security feature it provides. The keychain services on iOS provide a means to securely store content such as passwords, keys, certificates, etc.
Sensitive data like passwords and keys should be stored in the Keychain. Apple's Keychain Services Programming Guide states that
"A keychain is an encrypted container that holds passwords for multiple applications and secure services. Keychains are secure storage containers, which means that when the keychain is locked, no one can access its protected contents".
Here is two nice tutorial for you.