The long term support version of Ubuntu.
Ranked in these QuestionsQuestion Ranking
Pro Great Long Term Support release schedule (2 years)
This allows for users to always have a new supported release available without long unkown wait times in between.
Pro Just works out of the box
Lots of support for hardware, lots of pre-installed software, and a smooth install process means less time downloading drivers, less time digging through configuration files, and less time deciding on software to use just to get up and running. It also means less time digging through forums looking for support.
Pro Good PPA repositories available
PPA repositories allow you to install the latest version of your preferred software while keeping the rest of the operating system "stable".
Pro Lots of support
As the most popular Linux distribution, there's a wide range of sources for support online if you ever need help, including the Ubuntu Wiki, Ubuntu Forums and the Ask Ubuntu Stack Exchange site.
Con Relatively high system requirements
The default Unity desktop environment is a resource hog which requires hardware accelerated graphics rendering in order to run smoothly, making out of the box Ubuntu unsuitable for low end systems and older hardware. Even mildly aged hardware, you'll get far better performance out of a lighter desktop environment like LXDE or XFCE.