I am building a web server, so I want to know what is the best edition of linux for that purpose.
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There is no "best". Debian is one of the most popular, however.
I'm guessing you're a beginner. I would say ubuntu server is the easiest to set up, and is based on debian.
By edition, I assume you mean distribution. I'd suggest debian for any server, not just a web server. Use lighttpd as you web server.
I'm fond of ubuntu server edition. Its based on debian, so many of the tuts etc are compatible.
It automates setting up apache and samba.
I've had good luck with ligHTTPD on my eeepc.
But then again, this is a local webserver and not intended to be used live on the web.
are you fishing for trolls? ;) In my opinion I found that useing an "enterprise" linux pays of. for me that is debian and if i need to run some redhat compatible software i choose centos. but i am sure there are other opinions out there.
In my humble opinion the "best" is whatever you are familiar with. In reality there isn't tremendous differences, between distros, in the applications you are going to be running on the OS (Apache, LigHTTPD, MySQL, PostgreSQL, etc.). There are a few exceptions of course, Lindows comes to mind (is it even around any more?).
Debian stable is great for a hosting on a VPS because it makes it easy to set up a bare bones installation and then add what packages you need as you need them. Many other distributions tend to install all the graphic kitchen sink that you don't need or want on a server, especially one as resource constrained as a VPS. I like the newer package manager gui "aptitude", which does a really good job of finding all the dependencies so you can just install what you want and it will find the things that need to be installed first.
I suggest stable for a server, unless you need real bleeding edge stuff - for instance, "stable" stayed on PHP4 long after "unstable" and "testing" were already using PHP5, but none of my sites needed PHP5 so that was fine.
Although, if you want to really get into the nitty gritty aspects of linux, I'd take a look at Slackware" it's at least traditionally known as one of the least user-friendly flavours. I've found this helpful in some cases, as it can foster a steep learning curve.
I assume you have Linux experience if you are putting together a web server. If you feel you are "up to the task" of hosting and managing a web server, you should have already acquired the know how to pick a proper Linux distribution. That sounded a bit harsh, but try taking your own temperature on the subject.
If this is a mission-critical system, you may consider a pay-for Linux host for a while until you are very comfortable with configuring, controlling, and updating a Linux console. If this is more of a self learning situation, any of the larger systems (Fedora, Debian, Ubuntu, Slackware) would be fine. Anything with great online documentation will pull you through. I initially made the wrong decision with getting a "how to " book on Linux. If you need a book, get a "how to administrate on Linux". I have always learned more from on-line documentation, since there's always someone to help you understand the material, and you can bookmark that information for later.
As far as security goes, you will always be safer on a Linux system with minimal, well patched services. Obviously only run what services you need, but a proper firewall will keep out the trash. Professionals will always have a way into your system, but that shouldn't stop you from being alert, and vigilant with log checks.
Good luck to you, I hope you do well.
Not Linux but FreeBSD is pretty good.
In my opinion, on a scale from good to best is: Debian, Gentoo and FreeBSD big words