I think that Now on Tap is the most useful feature. Tried it on a preview build while it was still active and it blew my mind with how much context can change how you search for stuff.
Better battery life and that media-stream-over-bluetooth bug to be fixed, it is really annoying to be listening to a good song and the song stopping for half a second just to resume after.
Google Now on Tap! Also I think my username will disqualify me from winning, so if this does get picked feel free to say "Charlie" won - 1,000 shot, but hey, you never know
As I don't have the twitters or the grams I'll just post here.
I think the feature that I am most looking forward to in Android M is better support for project Fi. I'm having a heck of a time with it on the T-Mobile ROM. Way more trouble than the folks on the M preview even.