Regular Ubuntu, whatever the version happens to be at the time. Last I heard we are on Natty Narwhal.
Ubuntu releases a whole new base OS every 6 months pretty much on the dot which is a free upgrade and it installs itself. None of the old software quits working for the most part unless Ubuntu decides to go with a different major system. If that is the case it does the whole change over automatically.
They did that between Open Office and Libre Office some time ago. All my icons stayed the same and everything just used the new program instead of the old one. Most painless productivity suite upgrade I ever experienced. I didn't even notice anything different until it said Libre Office when I opened a word document.
People used to windows may be lost for a couple minutes while they try to find everything, but its not hard or anything. Media players still open media files, productivity apps still open productivity files, web browsers still open web pages, etc.
I use Chrome on Windows 7 and Chromium on Ubuntu and it looks exactly the same.
Anyway, I think the biggest problem is just that people get used to doing things in a super hard round about way with Windows and they aren't used to how easy things are to do in Ubuntu.
When you want software in Windows you have to go on the internet and try to research a media player that won't give you viruses and you have to go to a different webpage to learn about a different media player or whatever.
On Ubuntu you just go to the software center and type in media player and it lists many of them that you can choose from, all of which are guaranteed not to have viruses in it.
You try to go out on the internet and download things with Ubuntu and you have so much trouble. If you even manage to download a file you won't be able to figure out how to install it.
Then you read that you are supposed to use the software center for this and you try it and its like... How did I live without this? Why doesn't Windows have this?
Anyway, anyone willing to try something different should try it. Worst case you can just delete it if you don't like it.
It is crazy how stable it is, though. I answer questions about BSODs in here all the time with Windows, but I never once answered a question about Ubuntu crashing or even ever heard of Ubuntu crashing.
Another crazy thing...
I have a DVR hard drive in my computer. Windows won't recognize the drive if I turn my computer on and boot into Windows. If I restart in Windows over and over it will never read the drive correctly.
If I boot into Ubuntu it will not only read the drive correctly without any errors at all, it will teach Windows how to read it. If I restart in Windows after booting into Ubuntu it will work in Windows and every time I restart Windows after that Windows will be able to read it without problems.
Then if I shut down, Windows will forget how to read it but Ubuntu won't.
Linux is a lot more error tolerant than Windows is. One error crashes Windows. If you have Ubuntu you don't even know if there ever was an error. Either there are no errors ever or you just never hear about it and the errors never affect whatever you are doing.
I could go on all day about it.
The differences are quite profound.
Just don't plan on doing anything that requires using Direct X because it won't work in Linux.
There is something called WINE that allows you to install Windows programs on Ubuntu and most of them actually work, though. There is a system that allows Ubuntu to intercept a Direct X call and translate it to OpenGL and display the graphics with OpenGL instead. That goes slower so its not great for some games, but it is pretty nice still.