As others have said, 2013 was clearly the 3DS' best year.

The OG DS is harder to pin down, especially since so many of use didn't really have a DS for much of its run. The DS was also the last Nintendo system to really not have software droughts, which means it was much more consistent in its output and the years were overall closer in quality. 2005 is the year the DS really showed what it could do, so I'd say then, but it's not so easy.