ArchLinux for it's easy customization of packages through ABS, this comes handy because many things, like kernels, are commonly patched with custom kernel hacks achieving great new features. The whole repository is built from ABS, so you can find the exact same packages and gain inspiration from them in /var/abs where all the scripts reside. Also it's general configuration /etc/rc.conf is a good and central place to configure your system and it's got an awesome wiki and forum. Most users come from Gentoo or Debian and those who like these distributions might also find Arch interesting. Lastly, AUR contains all popular packages that are not widely available in the largest distros repositories also a really great addition.
Debian for it's stability, it's tight-bondling between packages (Hated or loved - because it raises security and stability/maintainability) and comprehensive pickup on old versions that are patched to fix exploits and other problems. And of course because it's not Ubuntu which sometimes bury's Debian where they should only give it credit.
Fedora because it's entirely free and rpm-based, for those who are seeking an RPM-based distribution I would highly bet my money on Fedora- it's also got a text-mode install so you can have the bare bones and install upon that with yum. Yum has also got a pretty intuitive syntax which I got to like pretty soon after installing it.
Ubuntu I think is not worth mentioning, primarily for two reasons. It's hyped, it's advertised, it's not a god or anything.. It doesn't do anything for it's users to find it EASY to use Ubuntu, because just try and look or search in their official wiki. It definetily sucks the info you get from the official site and it talks for Canonical straight away, they are into promoting not into making Linux easy just because you can put together a distribution which has most "common" hardware and suspend/hibernation support. It's simply not good enough to help Linux onto the desktop. To Linux to reach the desktop you will need more than a guy with millions to give free cds around - you will also need a great place to house the information and guidelines for using this distribution and this is where Ubuntu inevitably fails if you ask me - Most of my friends using Ubuntu tell me they use third-party resources such as ubuntuguide.org - and on ArchLinux for example, everything you need to get a full-blown system including the bells, whistles and the programs that you never thought the wiki should take care of are actually already there! I've several times searched for a guide for things like SAMBA (Windows Sharing on Linux) when I then later discover that ArchLinux has one of the greatest guides at setting up Samba !
Lastly I must say- if you want to benefit from Linux, you will be happy when get to knowing it's internals. If you are seeking an OS that is just point-and-click then honestly, Linux is not for you. Not because it's bad, but because developers and maintainers of distributions simply cannot handle the massive amount of problems that appears when lots of small new devices appear that are only supported in Windows by standard- also most people are aware that Windows ALSO crashes now and then, the tendency is that people have used Windows for several years and knows the case already EVEN if it's not a logical solution they have all these problems facing Windows on their spine from several Windows relationships through time of getting a new computer, setting it up, helping others etc. Windows will also not work without a driver, and sometimes the bundled recovery disks are the only way to actually get a device to work because the laptop is either discontinued on Acer (I've tried that actually) and no proper driver is available for download. This is the same case as with Linux sometimes- only that you will not HAVE to install your bundled recovery CD, you just have to go look for unsupported drivers made by volunteer individuals. Simply a distribution cannot guarantee anything on new devices or chipsets that are only made for Windows and neither would they like to support it for "free" most people would like the initiative to come from the hardware manufacturer themselves, the latter is a last-resort.
But when you get to use Linux you will see that Linux's general commands are actually pretty great and funny to play with, they can do ALOT of things just by combining what is already there.
is a good example. Here people combine shell commands present on a Linux system to achieve funny things like connecting through SSH and talking out another computers speaker.
Another great feature is that you can mount iso's from the official GNU "mount" command. A thing I think you would never see on Windows now that they only got to making a "Burn Image" function in Windows 7 on ISO's it'll probably be a while before they will allow ISO/Optical device emulation.
Go explore, but choose wisely- with a little help from a friend or someone who is not rude to you on the IRC or maybe here (on LQ) you will be absolutely fine. With basic-to-intermediate knowledge of computers everyone should be able to complete a guide like ArchLinux Beginners Guide because it has so many points-of-view and great notes ..
Don't be scared by commands, there's a reason why all these variants of Linux pop up- because simply no company in the world will support Linux and take it under their wing or claim it is theirs when it is not. If Apple took Linux under their wings what do you think would happen? No computers would be sold that was not 100% compatible with Linux and shipped with an Linux OS with the infamous recovery disks so you can always count that your system will work the way you intend it to do and that all peripherals like webcams etc. will work out of the box simply because it's assembled and packaged from a hardware manufacturer that cares for Linux.
The problem is only, no one cares for something that's free in a corporate world where everyone is filled with lies (Linux is not more secure than Windows, it's just a matter of popularity before it gets there) only computer enthusiasts see the potential because they already can see beforehand that it has an enormous and highly intelligent user-base. And it's hard to create the perfect OS in the spare-time while focusing on what the end-user thinks if you are already known with the internals and have no troubles with any terms not known by a Windows user.