The Office UK is the original one, and the best IMHO. You have to watch it first, before you try the US version. If you don't understand any of the cockney slang, don't worry, it's all explained here on their website :-)
http://www.bbc.co.uk/comedy/theoffice/interview/gervais_explains.shtml

The US one is equally good too. You might want to give up after watching the first 2 eps, but don't. It heads off in a different direction and you'll stop comparing the 2 after a few episodes. It's run a full four seasons and gotten better with each episode. Too bad they've stopped due to financial constraints. I think the last episode was a web only release (webisodes??)