According to a report by Apteligent (via 9to5Google), it has been found that as far as OEMs releasing updates in a timely manner are concerned, Motorola is the quickest. This is the same report that found that Sony’s Xperia phones are the most stable. However it should be noted that this data is based on Android 6.0 Marshmallow’s release in the US, so while it might be true for US users, other parts of the world might beg to differ.
Coming in second place would be LG, followed by HTC, and with Sony, ZTE, and Samsung lagging quite far behind. However that could be changing this year as LG is already preparing to launch the LG V20 with Android 7.0 Nougat, and last we checked the company is already preparing an Android 7.0 preview program for LG G5 owners.
Samsung is also working on its release speed as they did state that they are planning to get Android 7.0 out on the Galaxy Note 7 within the next 2-3 months.