Well, Dropbox works just fine if you have the database less than 2gb (limit of the free account). OneDrive is a bit worse in my opinion but it's 5gb. If your db is more than 5gb then I'd really suggest either self-host Nextcloud or consider the cost of Joplin Cloud. The reason is that WebDav services (of basically any provider) are pretty slow and with large databases it makes sense to move away from remote WebDav at all.
Also, it is not a requirement, but I would recommend to use a separate cloud service for Joplin alone: if you use a native sync client of a provider, it may casually interfere with Joplin's sync (or even get confused and delete few crucial db files) which in turn may result in corrupt database out of nowhere.
