There are many to choose from, but many people would say the discovery of the Higgs bosun or 'god particle' is one of the top events, not only of the year but of the 21st century so far. However, this is also the year when the Curiosity rover landed on Mars, and generally 2012 has been a year of great scientific moments - you can read more about these here.
As far as news coverage goes, the 2012 Olympics was a huge event, and was special because of the breakthrough both of women athletes - now more prominent than ever before - and the massive success of the Paralympics, which has changed many people's attitudes to disability for ever.
For world and political events, the Syrian civil war, although the unrest began in 2011, has been one of the major stories of 2012 and will probably continue to dominate in 2013 as more outside nations get involved.
(The financial crisis is still a huge problem facing the world, but it's more ongoing than an 'event' so I won't include it.)