Anker Multi-angle Portable Stand
If your looking into a good desk stand for the iPhone 6s Plus, consider the Anker multi-angle portable stand. It works very well holding up the stand in both landscape and portrait mode.
I have the tech21 case on my iPhone 6s Plus and the stand holds the phone just fine. There's no hole in back, so your not able to change the phone when you have it in portrait mode.
The only thing to be aware of is that the stand weight is 6.1 ounces which is a bit heavy for a portable stand. In comparison, the iPhone 6s Plus weighs 4.55 ounces. This would be an issue if you plan to take the stand to use on an airplane.
I did use this stand when I was traveling and found it to be very handy. The weight didn't bother me too much, and I didn't have to worry about the phone falling out of the stand during some heavy turbulence.
Eskimo Crabble
A few years ago the Seskimo Crabble was all the rage. On YouTube you can find about 500 videos of people unboxing and reviewing the foldable adjustable phone stand. I found this brief explanation on another website:
The Seskimo Crabble will hold the iPhone for you when you are watching the movie with it. Then you will have free hands for your popcorn. Watch in portrait or landscape, this stand can be folded flat and keep to wallet like a credit card. The silicone rubber on the holder gives extra stability in place like plane or train. The Seskimo Crabble is available at $5.
It appears that the company has folded, as the seskimo.com domain point to a "this domain is for sale" page. You can still find the Crabble on eBay or various outlets. I happen to have one and tried it with my iPhone 6s Plus:
The stand is not very secure and the phone would probably would fall over if I was to use it on an airplane tray. It's good for a desktop, but you only have a couple of angle choices.