It depends on what you want in your system, and really there's three schools of thought here (and this is true for both hardware and software)
Firstly, the mainstream as far as most folks on SF go - you want something you know will work, you want support and you want it now. In this case, going with redhat based systems (RHEL gives you excellent support, and centos is a community rebuild of the well tested RHEL distribution). You however will not get the latest and greatest. In many cases this is also true of hardware.
The second is the 'middle of the road' point of view, which is the middle ground - going with something like ubuntu. You want new packages (at the slight expense of utter rock solid stability), you want an installer, and nice things.
In some cases people do run into trouble, but you have newer packages and things are reasonably tested. While there's a lot of hatred for Ubuntu here, its a good compromise between ease of setup and reasonably new packages. Debian probably is a slightly more conservative choice. These days, you can even set up Ubuntu with a low latency kernel out of the box. I tend to feel ubuntu and debian work for me, but ymmv. A lot of places that deploy a lot of servers like facebook and google go for this option.
Finally there's source based distributions. Initial setup in most cases is an utter pain in the rear. You make a mistake with setting up your kernel? Oops, spend a few hours recompiling. You don't get an installer either - thats for n00bs. You often get bleeding edge applications, and the option to compile them as you need them (which includes being able to pick optimisations for speed or memory use for example), and a rolling release. If you have very specific, esoteric needs, gentoo's great. If you need to roll out a few dozen systems and want to automate it... good luck. Source based distributions simply don't scale as well. You're getting a lot of flexibility, some* extra speed, but not maintainability at the same level as a package based distribution IMO. You're not likely to get 15% extra speed, and you'll likely end up wasting time trying to tune the compilation flag for your hardware, and if you mess something up, spending time working out what exactly failed.
The BSDs are a separate family of OSes. Some folk swear by them (at least one comms room regular is a freebsd user), and different BSDs have different focuses - for example openbsd is security obsessed, and freebsd is the 'mainstream' one. They may not, in some cases have the same kind of hardware support linux does, but that depends on quite a few factors.