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.

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.

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

Mondrian is the best code review tool I've ever used, but I'll admit that could be damning with faint praise... I'm very curious to hear what the better options are, because we use Github at my current company and I would go back to Mondrian in a heartbeat.