I'm surprised Reviewable[0] hasn't come up in this discussion. It does a great job of allowing stacked code reviews and even handles rebases nicely; the reviewer sees the diff between commit #1 and commit #1' (prime = after rebase).
CockroachDB[1] has been using it since very early in the project.
I also switched my dev team to it once it grew sufficiently large (at a now acquired startup). The primary thing it enables is actually having a structured conversation about your changes, and having several _revisions_ of your changes through which the reviewer can navigate, much like Gerrit or its progenitor - Critique. Once you experience that, there's simply no going back to anything else.
The UX makes me want to gouge my eyes out with a dirty spoon, but it's still worth it, in spite of the UX. I'm baffled why MS/GitHub can't just go ahead and rip it off at this point, to replace its own utterly atrocious PR review workflow.
CockroachDB[1] has been using it since very early in the project.
[0] https://reviewable.io/
[1] https://github.com/cockroachdb/cockroach