Just curious.  I think mine may have been the Lenses.

Tungsten theme.

Probably the Tungsten theme. it really felt like it changed the whole mood. Allowing me to work longer hours without being eye fatigued as I did when using Mercury.  However I cant remember if drum replacer and Vocal synch were released earlier last year. those are 2 tools that have really made life a lot easier.

Aux tracks

Were LP EQ and Multiband in 2016?

Bug fixes.
Especially finally being able to Ctrl+stretch an audio clip without snap offset changing in the background.

It's a toss up between Synth recording (internal) and Aux tracks. I find both very helpful.

I only upgraded to Platinum in August, so I'm not sure when the different features were first introduced. My vote goes to Aux tracks, assuming that they belong to 2016.

In my studio It's officially a tie between Melodyne Studio 4.1 Multitrack Support and Plugin Load Balancing

Melodyne integration for me too