In terms of both Physical Software and Hardware sales 2016 is the worst year on VGC (data from 2007 onwards). Nintendo really were dying until 2017, they sold less hardware and software every single year for 8 years in a row. If Switch didn't take off they were done.
2015 was the second worst in hardware and the third worst in physical software, (2017 was slightly worse). So yeah I think it's easy to say 2015 and 2016 are Nintendo's worst years in terms of sales.
In terms of game releases that's subjective, but I'm not particularly fond of there game output, though I think a lot of games I didn't like simply because I had to play them on the WiiU.
2008 was the best year for both Hardware and Software.
Predictions (Made July 2019)
LTD: PS4 - 130m, Switch - 110m, XBO - 52m 2019 : PS4 - 15m, Switch - 18.8m, XBO - 4.8m 2020: Switch - 22m (Peak Year)