Android 6.0 "Marshmallow" will be the next android version which was most likely to be released in Q3 2015. The third development preview has also been released. Android Marshmallow or Android M was first unveiled at Google I/O on May 28, 2015, Marshmallow will primarily focus on incremental improvements and other feature additions.
You can visit this link to Know more about the new improvements and additions to the new version of the OS.
When Will I get it?
Google Nexus Devices:
Users Google Nexus 5, Nexus 6, Nexus 9 and Nexus TV can actually use the developer Preview of Android Marshmallow. Go to this link to know more. You need to have an unlocked bootloader to this. But for other Nexus users, you can expect the update as soon as the final release is done. So you will the first along with Android One users to get this update.

HTC Devices:
HTC One M9 and M9+ have confirmed that the update will be rolled out as soon as the final update gets released. More devices will be announced soon. Stay Tuned!

source www.androidguys.com
Motorola Devices
Usually Motorla Devices get quick updates. The same can be in the case of Android Marshmallow. Mostly Moto X devices (2014 and Play) may get the update quicker.
LG Devices
Most likely LG G4 may get the update quickly than other models as it is LG's current Flasgship device and it surely has the specs to run Android Marshmallow.
Other OEMs
Samsung has a very bad track record of rolling out quick updates to their devices, but since there are rumors that there will be update for Samsung Galaxy S5, S6, S6 Egde+ and few other models. There is currently no info regarding the status of Sony Xperia devices right now.
So do you want to try android marshmallow first? Then you probably want to have a Nexus device or an Android One phone as they are the first to receive updates first.
Now, you'll have to wait till the end of 2015 to get the update on other devices.(Can be delayed even further!)
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