We have been working with Red Hat (yeah, big enterprise), however I'm inclined to advocate for a switch to Amazon's Linux. Like to hear others perspective.
At my day job, we run CentOS 5 on EC2 instead of Amazon Linux because we actually run a hybrid local-node/EC2-cluster and it is easier to verify software builds on a single platform rather than two of them. The distribution we use actually doesn't matter a whole lot, though, as long as it is consistent between what we can install locally and what we can install on EC2. Our choice is actually constrained by what Amazon offers for the node types we primarily use (cc1.4xlarge).
I use CentOS on my personal server's largely because I use Red Hat at work, In past four years twice my servers were compromised coz I didn't applied security patches on time. Personally I like Ubuntu and I used it on my laptop for couple years until I switched to Mac, Installing patches, apps and upgrades are very simple on Ubuntu compared to CentOS, if you are a new bie I recommend Ubuntu.
Ubuntu, largely because I came to Linux via Ubuntu and understand its idioms. Over time, I've noticed that documentation for most open source projects includes Ubuntu, which helps.
I've got a few servers personally that I run, all are either Debian or Ubuntu. That's what I run on my laptop and desktop, so it makes it easier for me to develop locally.<p>With virtualenv, github, fabric, etc., deploying isn't as much of a challenge as it used to be, but I like to keep it as straightforward as possible.<p>For the day job, we use RedHat Enterprise, because our target market is enterprise, and that's the most widely accepted Enterprise Linux distro in the enterprise.<p>I'm not a particularly big fan of RH-based distros, but I vastly prefer RH to Solaris or Windows, so I count my blessings.