There is no good method for comparing commits, and add comments within a Pull Request Review.

Reviewers could not Diff of any of the commits associated with the Review. They should be able to compare Commit #1 to Commit #3 or Commit #2 to Commit #3 and add comments, etc. This is helpful for developers want to see are the changes since the last commit and not total changes in the 'Diff' tab.

1) On the "Diff" tab, Commit #1 and the latest commit are always compared. This does not help when all you want to see are the changes since the previous commit. Again, when someone added a new file and had multiple commits for review, it shows as the new file added and all lines highlighted as green in the 'Diff' tab.

2) Still on the "Diff" tab, you can click the "full source" button. On this tab, you can review commit history and do compare any two commits. This is exactly the type of compare desired. However, you cannot make comments on this page.

3) On the "Commits" tab, you can see a list of Commits associated with the Review. If you click one, you will see a compare of that Commit and the Previous Commit. You can make comments on this page. However, the comments are associated with the Commit and not the Review. The comments do not appear on the "Overview" tab or in the Activities Log. So if anyone was to look at the Review at a later date, they most likely would miss these comments.
We do not want to flip around tabs to achieve this, also need to capture every review comments involved as part of Pull request record.