I sometimes use a little function [`MaxBy`][1], made to be analogous with [`SortBy`][2]:

    MaxBy[list_, fun_] := list[[First@Ordering[fun /@ list, -1]]]

You need the largest element by length, so you can evaluate

    MaxBy[data, Length]

Note: this is based on the same principle as @Brett's solution, but it is slower. @Brett's and @R.M's exploit the fact that Mathematica sorts by length by default, while my solution explicitly uses `Length`.  I still think it's a useful little function, so I shared it again. 

The problem with `MaxBy` is that it only returns a single element, while there may be more than one list of the same length.  Here's a somewhat slow but simple implementation that returns *all* maxima:

    allMaxBy[data_, fun_] := Last@SplitBy[SortBy[data, fun], fun]

  [1]: https://mathematica.stackexchange.com/a/905
  [2]: http://reference.wolfram.com/mathematica/ref/SortBy.html