I recently went down this route myself so can speak with some authority on the subject. I initially bought an arm from Amazon. I don't remember the model, but it was cheap, though sturdily built. It was also well reviewed. Long story short, it wasn't strong enough to support a 27" iMac. They're heavy bits of kit! Even with everything tightened to the max, the iMac would sink down onto the table as the weight was exceeding the loading.
In the end, I decided to get serious and bought a Humanscale M8. This is a serious bit of kit and super strong. Very well made from a company that makes wonderful equipment. I also have the M2 which is for lighter weight monitors and I use this with a 4K LG monitor and a second machine. The M series arms are worlds away from the cheaper arm I bought previously from Amazon. I got my Humanscale arms second hand on eBay and they've been great. They move much more more smoothly and once setup, they just stay put. The rotational axis isn't perfect, but it's fine once adjusted.
Another thing to consider - the VESA mount/mounting option. I originally intended to use a iMac VESA mount and it was only after attempting to remove the iMac foot (stand) that I discovered that not all iMac's can have the stand removed. On my iMac (the iMac 5K late 2014) this wasn't possible. I could have sent it to Apple but I occasionally use it on location so that would have been difficult as sometimes I
need that foot. In the end I settled for one of these
Humancentric Mounts. (Note not the same high end brand as HumanSCALE). It's a bit ugly and pernickety but it works fine and I have limited complaints now it's setup.
A few other things to consider, say:
- I have now moved to an iMac Pro and the same setup is working admirably.
- Since I bought my Humanscale arms it appears they have upgraded them to new '.1' versions.
- They have also added an ultra heavy duty M10 model.
- The iMac Pro was at the centre of a number of reports criticising the robustness of Apple's OEM VESA mount kit. Be wary of Apple VESA mounts at the moment.
Hope that helps!