What’s “best” is up for you to decide.
Migration assistant can allow you to be “selective” but will it be granular enough for you? Only you can answer that.
manually copying files over is very granular, but again, is it the right choice for you? Again, the answer is the same; you must decide if the time required is worth it and you also need to decide how you’re going to handle archival storage. Leaving it on an old macOS drive isn’t practical.
Backup to DropBox is no different than backing up to an External USB drive. You have to decide whatif this strategy is applicable to your needs, but asking about “DropBox” is like asking whether a socket, wrench, or a locking plier is best to remove a fastener; they’re all tools and will work best depends on your requirements.
Personally, I build a strategy around “data availability” so that i can be working with my data no matter the platform (Mac, Windows, and FreeBSD). for me, this involves utilizing the cloud as a key component in data storage
Also have a look at the following for more information: