If you want to get smoothest stitching on the Xiaomi Mi Sphere, I recommend MiSphere Converter, an app that was previously available only for Android but is now on iOS.
Xiaomi Mi Sphere (reviewed here) is one of the best 360 cameras for photos and virtual tours, and already has good stitching thanks to its very thin profile which minimizes parallax stitching error. Mi Sphere Converter is a 3rd party app that improves the stitching quality even further in several ways:
1. Optical flow stitching
2. Correct color difference between two lenses.
3. Automatic horizon leveling. You can also fine-tune the horizon level.
To use MiSphere Converter, you need to transfer the unstitched photos from the camera to your phone. There are several ways to do this (see the Mi Sphere FAQ). After the unstitched photos are in your phone, select them and “share” them to Mi Sphere Converter. This will open the photos in Mi Sphere Converter. From there you can calibrate the stitching (you need to do that only once for a camera), adjust the horizon, or choose how photos will be stitched.