Either "two thousand and ten" or "twenty ten." It's actually rather difficult to get the nuance of this one exactly right. For instance, in a story-telling context people tend to say: 'twenty'. Like when the narrator reads:
"In the Year 2525 the first Earth Cylon War broke out."
Here the narrator would definitely say "twenty-five twenty-five." Why? Because it sounds more interesting. No real grammatical reason. Yet it's harder to find someone who says "In the year twenty-ten (2010)." As a vague rule of thumb: the closer to the current year, the more people use the 'regular' way of pronouncing years; like: "This is the year two-thousand-(and-)ten." Yet, the Russian Revolution started in nineteen-seventeen, and not so much in nineteenhundred seventeen (unless you'd be talking to a historian again).