What render settings should I use to get the highest quailty for my animations?
Those are some traditional tips we have in the wiki:
wiki.synfig.org/wiki/Doc:How_Do_ … quality.3F
Also, with ffmpeg target you can achieve as good results as the selected video codec and the video bitrate you use. Since Synfig 0.63.00 there is one target dialog for ffmpeg paramerters that is available at render dialog when ffmpeg target is selected.