OLED is certainly the technology to watch, but only LG and Sony are really investing into it. Samsung are going with QLED, which is basically just a slightly upgraded LED TV, as opposed to OLED which is a completely different underlying technology.
In terms of OS, I prefer TVs that use Android TV due to the fact that it is more likely to get updated for longer, as it tends to be that with most of the work being done by Google, the TV manufacturers are willing to release updates for longer. Samsung are quite poor for updating their TVs in the long term.
4K is certainly a technology to invest in, with HDR being a nice addition but not a necessity. 4K content is definitely going to become more ubiquitous over the next year, with Sky being one of the first to release mainstream TV with 4K soon, which means the others won't be far behind.
In terms of the built in Netflix button, I think they have struck that deal with several manufacturers, as a Sony television my grandparents own also has a dedicated button.
At the moment my recommendation would be an LED 4K TV from Sony, LG, or Panasonic. Samsung TVs are good, but as I say, their smart features tend to get left to die, meaning it is more likely they would be vulnerable to security flaws. Sony tend to be the sharpest, while LG tend to be the brightest and most colour accurate.