I want to take LPI exams.
i recently used redhat/fedora linux
i currently using openSuse Distribution.
i want to switch to a linux for professional reasons.
which of following distributions is best & suitable for me?
and which is most popular than other distributions? (both Desktop & Server)
-SUSE Linux Enterprise (SLES)
-Redhat Linux Enterprise (RHEL)
If you want an absolutely middle-of-the-road Linux, choose Debian. It's about as close to being "just Linux" as any distro can be. It has huge software repositories and gives you the choice of stable or bleeding edge software. It's also very popular.
If you want a learning experience that involves really getting your hands dirty, choose Gentoo or Slackware. They make you engage with the internals of the OS.
The distros in your list above are all very popular distributions. You can check the link in my signature for lots of good information on getting started with Linux. You can also find a couple of quizzes there that might help you to pick a distribution.
In the end, the best distro for you and your computer hardware is the one that you've chosen yourself as the best, so it generally takes some experimentation along with some trial and error to find the best.
Do let us know what you go with and how it all goes for you. :)