Specifically for Harmonious, it can be viewed from anywhere around World Showcase lagoon.
There are fences all along the water, so many people find it works well to have the person who needs more ‘space’ right along the fence and have the rest of the group alongside and behind so no one can come within the ‘bubble’ they created.
You can look for a possible spot during the day - just look for all the barges and decide what kind of view it would give.
It will be busiest and most crowded the closer you are to the World Showcase entrance or the International Gateway between France and the UK. Also keep in mind that the terraced areas by Italy and France are often roped off for use by parties.
If you don’t care about being right up by the water, the bridges by the Odyssey Building (near First Aid and Baby Care) give a pretty good overview and are usually not crowded. If it’s too noisy, you can also retreat into the restrooms right there.
When leaving, we always wait 10-15 minutes before even trying to leave. The first part of the crowd usually starts walking out before the show is even over. Waiting a few minutes means a leisurely walk with a few people instead of a hectic walk with the crowd. And, if you are using the buses, the first packed buses will likely already have gone