To what format should I convert video to preserve the best quality possible?
Everything depends on the device you would like to watch your video on.
For conversion of HD/FullHD videos and viewing them on computer we'd recommend you to use the appropriate format from HD video formats' list - either format with 720p or 1080p quality.
If you are going to watch this video on a portable device and would like to keep the file size low, choose the appropriate device format from the list of formats for Devices.
If you would like to use DVD-player to view the video, it's better to select one of the formats from DVD-Compatible video formats' list - either PAL- or NTSC-compatible format.