I have been using FreeBSD and Linux for about 10 years now and I'm opinionated enough to give mine if you like:
First, in Linux, Bash might be the default, but it's not the only one. You can install any you wish to on it. On my Debian, and OpenSUSE machines, I have about 12. I have Bash obviously, but also Csh, Zsh, sh, pdksh, ksh93.... I have a few that are like Perl stuff but I'm no coder so I don't use them much, and I also use fish and the others. Usually my real choice comes in which terminal emulator I want to use, I mean Xterm is pretty standard but I've always liked E-Term and W-term more. I also use A-term. I generally install all of them so I can pick whatever I like.
In FreeBSD and BSD in general, Csh has been standard, and I do use it, but normally I like Zsh more. And I use Bash too. This is what I did after I installed:
pkg_add -r bash zsh Eterm wterm aterm (You get the idea).
Anyway, the main point, which I think everyone else brought up too, was that the Shell YOU like, is the best one.
That doesn't exactly answer the question you may have been asking though, because I think you might have meant which ones everyone uses and why, so I can add that a bit too:
Bash - This is standard on Linux, and available very much so on FreeBSD, and a lot of people seem to like it because it has a lot of features. You can use tab completion and so on, and stuff that other people really like. Bash scripting also seems pretty popular in Linux.
Csh - Default for BSD, available for Linux too, and it's still good. I don't know enough about this to really give you much info, but I do know the history behind it, and if you're coming from Linux, and what history, which I recommend because I like it, there are two movies / documentaries I'd recommend for you - "Revolution OS" which is more for the Linux and Gnu / FSF world, and talks about all of that, and then there is a DVD I bought from the FreeBSD Mall "20 Years of Berkeley Unix" which talsk about the history of BSD and Unix in general, and MKM is the speaker. He's AWESOME! He's VERY entertaining, and he is a great speaker.
I also have the book "A Quarter Century of Unix" which my Wife bought me this past year for Christmas. I LOVE it. (She also got me the book about Linus Torvalds the year before, ahh the joys of having a Unix knowing Wife).
Those are great by the way.
Anyway, the PDKsh, Public Domain Korn Shell, that's not bad either, but again, I'm no expert with it, but there is a HUGE book about Ksh scripting and other stuff that my Wife actually had from before we even met, and it's seems interesting, and seems to have a lot of it's own neat Features.
Zsh - I usually install this on everything because I REALLY like it. The first time you run it, you do a quick configuration, which is pretty easy, and not required just to run, but you get to set things up how you like, and it has a lot of the same features Bash does, and it's in general really neat. I don't know why I like it so much but I do.
What versions of Linux did you use? On the install disks for OpenSUSE, you can install everything from Bash to Zsh, but on some others, they don't even include anything but bash. Debian allows you to apt-get all of them though.
