Which game editor is most widely used by game companies or asked by them when seeking for a job, UnrealED, QRadiant etc... anyone else?

Each of those is specific to a game engine. If a company were making a game based on a particular game engine, it would probably want level scripters who were familiar with the editor for that level. If the company were making its own engine with inhouse editing tools, none would be useful skills. Ultimately, the most important skill is probably (a) good general level development skills, and (b) the ability to learn new tools and technologies quickly.

Of course you are right. What about Blender? Is it more generic? Anyway, from all these editors which is the best and easiest to start?

I'd say that being able to work with multiple tools was more of an advantage. If anything companies seem to use pretty expensive off-the-shelf applications like 3DSMax and Maya. I don't think either QRadiant (I think GTKRadiant [It's a development of the QRadiant codebase] is better though) or UnrealEd are difficult to use.

I've used GTKRadiant quite a bit, the problem isn't really in using the editor, but ensuring the level works - that it feels nice to play, plays well and looks good - and has (preferably) no bugs! Getting area size and fitting stuff together so stuff 'flows' is a real challenge!

Both GTKRadiant and UnrealEd are worth learning though. Quake engine games tend to use Radiant (Quake3, SoF2, Elite Forces, RTCW, Enemy Territory etc.,). Unreal Engine games (Unreal, UT UT2k3/4, America's Army etc.,) use UnrealEd. They're both commonly used 'big name' engines.

Level editing seems to be a valid way into the industry (if you're are good enough. I think there's a few people at id who were employed because of the free [it's a licensing condition] levels they made.)

Good Luck.

It's the general skill that are important, start with the game you own (If not Enemy Territory is free - as are the tools!).
The tools normally seem to be free to owners of the game.

Enemy Territory: http://games.activision.com/games/wolfenstein/
GTKRadiant: http://www.qeradiant.com/

Note: GTKRadiant (and probably UnrealEd) is only free for non-commercial use.

Blender is a 3D model editor, not a level editor. Level formats are often very specific to the particular game.

I think working with previous games from the particular company is a good way to go. I know a couple of people who got hired as level designers in new RTS games after creating some amazing levels in Age of Empires II and posting them to various fan sites.