I use 16:9 aspect ratio to watch everything on Roku, I heard that 16:9 aspect ratio may not be the best because it zooms the image or scales the image and some people recommend setting aspect ratio to "Just Scan" So which one is better for Roku, 16:9 or Just Scan?
The silly putty effect of stretching the width of 4:3 ratio video to fill a 16:9 screen should never be an issue with a Roku.
You should only need a Just Scan setting for legacy devices which connect via RCA (composite or component) or S-video cables. A legacy device needs to be manually told if it is connected to a 4:3 or 16:9 device (if it even has such a setting even available) and usually the responsibility to add pillarbars black bars on the left and right sides to correctly display 4:3 inside a 16:9 frame is left up to the TV to perform.
When connecting via HDMI, the device can detect that the TV has a 16:9 aspect ratio via EDID (Extended Display Identification Data) which wasn't possible using legacy methods of connecting. Once the Roku detects via EDID it is connected to a 16:9 aspect ratio TV, the Roku should take responsibility for adding pillarbars itself when it detects a 4:3 ratio stream.
May be for Roku 16:9 is best but for Amazon fire stick and Xbox One I think Just Scan is better. For example on Xbox One they have TV Calibration tool which lets you set the correct aspect ratio and Just Scan seems to match what the ideal picture should be
I misunderstood what you meant when you said zoom/scale. It sounds like the manufacturer has decided that 16:9 mode should also include performing overscan and that "Just Scan" is the terminology used to make sure overscanning is turned off. For the Roku, you want it set the same as the Xbox One so choose "Just Scan" for that TV. The concept of overscanning was a work-around for limitations with CRT and project based TVs that really should have ended when LCDs gained dominance. For anyone that doesn't have a Xbox One, finding any BluRay disc with THX Optimizer also can assist in determining if the TV is performing overscanning for HDMI connected devices.