How secure is iCloud email address? Compared to AOL or GMAIL?

It is as secure as your password. Choose a strong one - not 'password' or your dog's name 🙂.

Copied from: http://support.apple.com/kb/HT4865

All traffic between your devices and iCloud Mail and Notes is encrypted with SSL. Consistent with standard industry practice, iCloud does not encrypt data stored on IMAP mail servers. All Apple email clients support optional S/MIME encryption.