DS: 2005. Mario Kart, Animal Crossing, Nintendogs, WarioWare, etc. This is the year where the idea of the DS being a "third pillar" died.
3DS: 2013; If we are talking about the West that is. Awakening and New Leaf were 2012 releases in Japan. They were the strongest 2013 releases in the West for the 3DS.
It would be interesting to see a similar thread about the Wii and Wii U peak years, and maybe the NES/SNES, and GB/GBC/GBA.
Lifetime Sales Predictions
PS4: 130 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 110 million (was 73, then 96 million)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima