Aside: In your opinion what are good issue trackers and code review tools?
As for the code review tools though, I've not yet seen something that I feel is materially better than GitHub + PRs + some bot-enabled automation. I understand that many people _prefer_ things like Gerrit, but I've found the difference to mostly be opinion, and that Gerrit can be a significant problem in getting contributions - it's quite hostile on first use.
Github reviews are fine for a one-and-done merge, but for a back & forth review I really wish it had the above...