At work I have used Backlog and Gerrit and would say Gerrit was pretty good. While you are doing comments you can apply it or not to save it. People would reviewed my code were just writing it in the comment: "Note to self: etc". The diff was also good with Gerrit, it highlights changes in a better way than others. I don't know if it changed or not though since now I am also a lone wolf using Gitlab.