I've no reviews or polls to review but I can share my opinion with you on production environments.
If you're running your corporate business where the big players are dealing around (IBM, Oracle/SUN, Microsoft, HP, ...) you've already seen a lot of big names: Solaris, Win Server, HP-UX, ... and Linux as well.... In these environments you'll probably see RedHat (RHEL, not Fedora) and SuSe more often, because you've a "stable" and supported package, you need some sort of vendor support (payed support I mean), you've hw tailored to your needs (IBM for example) and you need a Linux distro "certified" for particular applications (Oracle db 11g, IBM J2EE, SAP, ...). Many hw vendors provides you their own linux distro or a distro closer to them (IBM with SuSe for example).
When you don't need (or you don't want) a payed subscription you've distributions officially unsupported by big vendors but totally compatible with those big players, CentOS is a good example of it (they're now providing payed support as well if you need it) but there're a lot of samples when you refer to the enterprise market, most of them are "RPM based distros"
When you're vendor independent and you simply don't care about hw or your software provider doesn't force you to use a particular distro (example: you don't have oracle pushing you to use a "certified distro" for 11g) there're a lot of good samples: Debian is preferred in a lot of server environments, it's stable, it doesn't provide cutting edge versions of your favorite software (you prefer to have an "old" but stable package instead of a new one in production areas) and it's still here from the beginning. Another good thing: major upgrade is easy and painless, .DEB package management is fine and easy.
If you deal with Research companies, universities, R&D departments, ... you'll probably take a look at Gentoo (at least here in EU), when you've to get the maximum from the resources you've this is a good starting point even if you've to master it for a while to be more productive; Arch is becoming more and more popular and it's fitting in the same spaces where Gentoo is.

That's my opinion but hopefully there're other comments as well
Let me know if you want to clear some aspects

Ben

Hi,

I've found that most popular for Enterprise class seems to be RedHat EL, SuSE SLES, and depending on the requirements of the environment, certainly for cloud environments many people are hosting using Ubuntu,Debian or Amazon Linux.

Also, I'm pretty sure you can pick up some stats on distrowatch.