The problem with putting it in the URL is that the URL is supposed to represent a resource by location. An API Version is not a location and it not part of the identifier of the resource.
Sticking /v2/ in the URL breaks all existing links that came before.
There is one correct way to specify API versioning:
Put it in the mime-type for the Accept: header that you want. Something like Accept: application/myapp.2.0.1+json
Chain of Responsiblity pattern goes well here especially if there will be significant number of API versions that are different enough to have to have their own handler, that way methods don’t get out of hand.