can anyone tell me which one is the best & popular cli os in linux & unix?Are you REFERRING to the shell?

BASH is the most common standard as far as shell's go. There are many other shells that can be used, however. While the wikipedia entry on the subject is not exactly the most thorough, it seems to provide a GOOD overview.

http://en.wikipedia.org/wiki/Unix_shell

As far as Linux goes, generally what you get with the distro "ought" to be what you use most often. This doesn't mean you cannot use another shell- far from it- but most distros contain a number of shell scripts for various purposes, and these will of course not always work in other shell implementations. Whether this constitutes a problem depends on the specific user and what they are trying to do, of course.i actually meant gentoo linux ,redhat fedora linux in linux family....
& solaris ETC in unix family... ,which one's CLI?CLI?
You mean Command Line Interface.
All Linux distributions have a CLI and they are also called 'shells'.

For the ENJOYMENT of all... Here is a video...
playing With Shells In Ubuntu