When it comes to clothes, I'm a normal guy. I just want to walk into a store, grab something that fits, buy it (What, try it on? Are you kidding!?), and head home. Well, that's what I want to do. I've learned over the years that just because something should fit doesn't mean that it will fit.
It's the same with Linux servers. Sure, they're all built on the same code base and can run the same applications, but one may fit you perfectly while another may make you look like a clown.
So, how can you tell which is which? Well, let's start with that basic question you should bring to any computing decision: "What is it that you really want to do?"
Corporate business use
Let's say you have a company with several hundred to several tens of thousands of users. What do you want? This one is actually a pretty easy call. Your first choice should be Red Hat Enterprise Linux.
Red Hat has big business support down to a fine art, and it's easy to find certified technicians, administrators and engineers who know their way around RHEL. It's also supported on a wide variety of hardware. Whether you're running x86 servers on racks, blade servers, IBM POWER systems or mainframes, there's a RHEL for you. In short, Red Hat is the gold standard of business Linux.
Is RHEL is too expensive for your taste? Well, you get what you pay for, but there are two other worthy business Linux distributions that deserve corporate attention. These are Oracle Linux and SUSE Linux Enterprise Server (SLES).
Oracle Linux is essentially a copycat version of RHEL. While I know a few people who prefer it to its parent distribution, most of the folks I know who've tried it find little to recommend it over RHEL. SLES, on the other hand, has a handy setup and administration tool, YaST, that I find very useful for setting up branch office-sized deployments.
Let's say you already have a real team of Linux experts on staff who aren't wedded to RHEL, Oracle or SLES. In that case, you might want to consider Debian. This is a community Linux, but, for those who know its ins and outs, it works extremely well. Debian is not, however, a Linux for non-experts.
Small business use
OK, let's say you have only a few hundred people in your organisation. Or a few dozen. Maybe it's just you and the dog. What do you do now?
Well, all the choices above are still valid. Here, though, I see it as being more of a dead heat between RHEL and SLES for your IT budget. In my experience, RHEL is easier to manage on numerous servers scattered across multiple locations, but SLES has always done well for me in small offices. I recommend trying them both and making up your own mind.
At this point, if you already have some strong Linux administrators at hand and you want to save some money, I'd recommend looking into Red Hat and SUSE's community distributions as well: Fedora and openSUSE, respectively.
What you won't get with either is support from their sponsoring companies. In other words, you're on your own with these distributions. But, if your IT staff know Linux well, you may not need help for the demands of a small business. For my own small office, twenty desktops and four servers, openSUSE works just fine.
What about Ubuntu, the brand of what may be the most popular of the desktop Linux distributions? There's Ubuntu Linux Server as well.
You could use Ubuntu Server for bigger businesses, but for enterprise-sized loads I prefer knowing that I have a company behind me, like Oracle, Red Hat or SUSE, that has lots of experience in dealing with data-centre sized installations. For a rack or two of servers (at most), Ubuntu should do just fine.