I want to create a user in RHEL 6.5 and i have three commands (1)useradd (2)adduser (3)system-config-users which one will give the complete user creation.
adduser is there for compatibility with older versions of RHEL, and with other OSes.
For purely command-line operation,
useradd is the preferred method.
system-config-users doesn't fit in the same category as the other two. It is a GUI program. It's implemented in terms of
userhelper, which has a command line I/O interface more suitable for use as the back-end to a GUI program.
(This according to its man page. Its man page also says it's not meant to be used interactively, so it doesn't replace
useradd for your purposes.)
Bottom line, use
system-config-users if you want a GUI, and use
useradd if you want a command line app.