So far HTC is the one of the first companies which has confirmed that they will release the new update to the One M8 model.
Other than this the Android One series- like Micromax Canvas A1, Karbonn Sparkle V and Spice Dream Uno would get the update in India.
Moreover, Android 6.0, Marshmallow is going to be running on :
These Sony Experia models:
Xperia Z2, Xperia Z2 Tablet, Compact, Xperia Z5, Xperia Z5, Xperia Z5 Premium, Xperia Z3,Xperia Z3+,Xperia Z3 Compact, Xperia Z3 Tablet Compact, Xperia Z4 Tablet, Xperia M5, Xperia C5 Ultra, Xperia M4 Aqua and Xperia C4.
These Motorola Models
Moto X Style, Moto X Pure Edition, Moto X Play, Moto G3, Moto G2, Moto Turbo, DROID Turbo, Moto X2, Moto MAXX and Nexus 6
so depending on the phone among these above list, you can get just any one of them which would fetch you this Marshmallow update.