My favourite linux distro is Debian.
I like it's stability... it's idealism and principles (as epitomised by it's Social Contract)... it's versatility (server, desktop, embedded device, vast number of architectures)... it's package management... it's independence from commercial influence and it's democratic decision making processes.
I like the way you can do a net install and start off with a very minimal system and just add what you need.
However, what is right for one person is not necessarily right for another.
For instance, Debian is renowned for it's stability, but part of the reason it is so stable is because it only includes older software that has been painstakingly tested.
If you like to always use the latest software then Debian may not be the best choice for you.
(You could use Debian testing but you should be aware that Debian testing does not receive security updates in a timely manner and also may not be quite as rock solid stable as Debian stable).
Arch Linux is a good choice if you want to always have the latest software.
Ubuntu and Linux Mint are generally thought of as best for beginners. (Most user friendly.)
I haven't used OpenSUSE for a long time but I know a lot of people have used it because it has had a good implementation of KDE.
As far as desktops are concerned I like them all in different ways.
Obviously xfce is more lightweight and faster than Gnome or KDE.
This could be a big advantage if you have a very old computer - perhaps won't be as important on a more modern, more powerful machine.
xfce is popular with those who like the traditional classic desktop. It is more minimalist than Gnome or KDE, both of which could be considered bloated. For me the disadvantage of xfce is that there are not many apps written for it.
KDE has some excellent apps. Digikam stands out for me as a fantastic photo manager. Amarok is a great music player. K3B is the best CD burning application I have ever used. I love the virtual folders CD ripping function in Dolphin/Konqueror... it is a terrific idea.
The plasma team, headed by Aaron Seigo, have done a truly fantastic job and have made KDE by far the most customisable and most powerful desktop around.
However, there are some weaknesses in KDE. Konqueror, once an amazing web browser, has basically stagnated for years, sadly and all of the advances in web technologies over the last 5 years or so seem to have passed it by. Kontact has the potential to be far and away the best pim (personal information manager) around... however, it is really buggy. I wondered if it was just me, but look on the internet and you will see that there are a lot of people out there who have found likewise. Horror stories of people losing their emails abound. Someone said KDE has really great ideas but lacks the manpower to be able to implement them all properly. I think this is probably the crux of it. Akonadi and nepomuk are fantastic ideas but have simply not worked as advertised and KDE 4 has been around for 5 years or so now, so by now these things should have been resolved.
The most important applications for the average user include web browser, email program/pim, office suite, IM with voice video chat.. All of these are lacking in KDE at the moment. KOffice was forked to Calligra which is still in early stages. KDE Telepathy is still in early development.
Gnome has a lot to offer. It has some fantastic applications. Simple scan, GIMP, Inkscape, Rhythmbox. However, I find the new Gnome 3 interface annoying at times. I miss the task bar. You cannot have widgets like in KDE or customise virtual desktops like you can in KDE.
Soundjuicer is no longer maintained and often cannot find the metadata for CDs. Otherwise it is a fantastic program, but this metadata retrieval problem is a really major problem unfortunately.
Evolution is the Gnome PIM and it has a lot to commend it... I particularly like the easy built in backup feature which Kontact lacks. However, Evolution too I have found buggy... especially the calendar part. Also the integration with Gnome Online Accounts is not in my experience as smooth as one would like.
If you had the best features of these 3 desktops then you would have a truly kickass desktop.
Ultimately, however, you need to try these things for yourself and make your own mind up.