Well I'd say W10M.
But I really can't use it at the moment for a few differents reasons :
Battery life : I'm using my phone a lot. And the camera too. With 8.1 I can actually take a lotta pictures in a day and will be able to get back home with some battery left. Not that lucky with 10
Responsiveness : I'm always busy, I need to answer quickly to my messages (mostly Sms/Messenger), and with 10 I was having real troubles sending anything from these apps. (And here again, taking pictures is REALLY and that gives a point)
Pictures : I know that the app isn't done yet. But it's a mess right now; when I take a picture/screenshot I wanna share it right-away. With W10M you'd have to open the Photos app to be sure the photo has been indexed and will be found when you will have to share it. With 8.1, you can take a photo/screenshot, directly open the app (WhatsApp for example) and browse to your picture.
Cortana : I don't know why but the Cortana on 8.1 is (or feels, idk) faster than the W10M version so far.
And finally...
Notifications : Multiple points here.
W10M is (often) struggling to detect that I have SEEN a notification. And it will still show in the glance for a few minutes.
And ofc... Custom notifications tones