Quad Lock seems to be the bee's knees. I have a Galaxy S8 and the Quad Lock for that isn't out yet. If it had been, I woulda got that first and been done with it. Might be a bit more expensive than the competition, but seems worth it to me. Seems wiser to go that route than to try several cheaper ones and still not be satisfied. I have a cheapie one right now that gets by. There's several versions found on Amazon. It has a spring loaded clamp for the sides, then rubber band type things that grab each of the corners. It's stable enough. I trust it for now, and for a couple of seasons. Yet I imagine those bands will eventually break. It does take some fidgeting to get it secured in to my liking without covering too much of the corners of the screen, or accidentally opening some apps in the process. It mounts to the handle bar securely, yet I'd prefer to mount on the stem like the Quad. Or at least the option.
The only thing I don't like about the Quad is having to use a dedicated case. Though the more I've thought about it, I find myself taking the case off my phone for several reasons anyway. So realistically, swapping or removing cases really isn't a deal breaker.