[bradfitz]

From best to worse, in my experience:

1) Google's current internal one

2) Gerrit (open source, to be used by Go)

3) Google's old one (Mondrian)

4) Rietveld (open source, but run for free at codereview.appspot.com)

5) Github

I would totally suspect that Phabricator or Review Board would be well above Github (as are 1-4 in my list), but I don't know where. I have little desire to use or explore new code review systems at this point. Four per day is enough for me at the moment.

[piotrkaminski]

In case you do want to try out another one at some point, I built https://reviewable.io to take some of my favorite features from Google's internal tool but integrate seamlessly with GitHub.

[maherbeg]

Review Board isn't as good as it could be. Sometimes it's diffing tool provides far too much noise to be useful.