Aux Tracks/Patch Points.
 
 
 
...And everything else.   
 
Seriously though, I do use AT/PP the most but have used so much of everything else introduced in 2015 I would never want to "settle" for just one thing.  Fortunately with SONAR I do not have to!     Bring on SONAR 2016...
 

Lets face it...2015 was a good year to be a Sonarite.  Like Backwoods said, I thought about not upgrading ever 'cause I was making music and everything was solid. But the Bakers kept adding cool thing after cool thing.
 

I remember the days when reaper users used to come onto our forum and boast about the regularity of their updates.  With SPLAT, we are getting much more significant updates/upgrades than they ever did.
 
Some of the monthly updates have included more significant feature enhancements than some competing product's yearly, expensive version upgrades.  I don't know how the bakers have been able to keep up the frenetic pace and how they can pack in so much every month or two.
 
All I can say is that the team at Cakewalk should be heartily congratulated.  You guys deserve a break - have an awesome Christmas!

As I deal mostly with midi composition of late, I would say the midi overhaul is tops on my list. 
In a photo finish 2nd are the aux tracks/patch points.  I use them on everything now! 
Make my mixing workflow so much easier and neater.
Mix Recall is 3rd (but not forgotten) as it gets more creative use...
 
Of course, every enhancement seems to bring more and more life to Sonar.  The optimizations were like a straight shot of adrenaline both for my machine and for my inspiration to just "go make music"..
And I think that's what you Bakers intended right?  
So, I'll get back to it..
Thanks for all the hard work, the landmark year and for continuing to help me make music!  

oh yeah...virtual controller... forgot about that.  Use it to get quick sound ideas.

Vocal Alignment, Speed Improvements, Solidity (Solidness?) (Solidarity, Yeah!), Patch Points.
The fact that after 12 updates, both my laptop and desktop didn't break once. I did have one issue with some FX Chains not working correctly, but sometime around Jamaica Plain, they too were just silently fixed and now work as expected. Thanks to this forum, Craig and support for their help on that!

Patch points and Aux tracks would top my list.
 
Thanks to the Bakers and Admin staff at Cakewalk for a fantastic 2015.  Have a great Holiday season.  You deserve it!

Just read this from Craig.
 
Performance Optimizations
Craig Anderton, Gibson Brands

If things run any smoother, I may have to cover my computer with grip tape.

Craig, that was a great comment. 

 

Mix Recall is far and away the best of all features and there's many to choose from.

Mix Recall, aux tracks,  patch points - everything basically!!!!