Hands down, one of these
https://www.ergotron.com/pl-pl/products/product-details/45-243#/
Not cheap mind!
There are others like the allwiner but found others can't take the weight like the ergotrons can.
Currently have 2 of them. One with a asus pb278q(27) and one with a sammy hg70 (32)
I've just bought 2 of these, I've never had an ergotron to compare quality but i can't honestly think what the ergotron would do that this doesn't to justify its price premium
ooops, competitor link removed
I've bought (for £25 each) 2 of these
North Bayou TV LED Monitor Wall Mount Bracket with Full Motion Articulating Swivel and Gas Spring for 17-27 Inch Flat Panel Displays
The gas arm lift strength can be adjusted to suit the weight of your monitor. the other "knuckles" can be tightened/loosened to suit how easily you want it to move.
It will go flat against the wall if you want it to. the arms fold over each other.Thanks, I already have one of these in my basket on Amazon, wasn't sure looking at it how close it goes back to the wall?
I may order one of each and see which is best for the cost.