* `git log branch` because I want to cherrypick or checkout parts of another branch.
* `git log --stat` because what files changed can be a big clue for what I'm looking for.
* `git log -- dir1/ file1/` because I only care about commits to a certain part of the tree.
Other than that, `git log` already provides so much information to /search or even `grep` through that I can't think of any other flags I use regularly, and if you don't use them regularly you forget them.
The real GOAT that people are sleeping on is `git rebase --interactive` where you can go back and edit part of your branch to clean it up before rebasing or merging towards main. The cleaner the commits are, the more useful they become later for other tools like log, merge, rebase, cherry-pick, bisect, etc.