Merge. One of the values of a VCS is to preserve history in detail, and merge is the only method that does that. Also, it's easy to foul a remote branch with the other methods if someone has already pushed changes to branch 1.
Shared branches should always only move forward. Most Git-* systems support stuff like protected branches.
I personally like tidying up your own feature branch with rebasing and then merging it into main (preferably using only FF merges). However this is not scalable for some larger projects, and for example monorepos also make this hard to accomplish. In those cases the solution ends up being squash+merge.
The extra information about the squashed commits is usually persisted to these systems (GitHub PRs, GitLab MRs, etc) so you don't really lose much, I guess. Although I do prefer keeping it all in plain git.
I feel it's a balance. Each operation has a purpose.
Rebasing makes sense when you are working in a feature branch together with other people so you rebase your own commits to keep the feature branch lean before you finally merge it into the main branch, instead of polluting the history with a hard to follow mess of sub branches for each person. Or when you yourself ended up needing to rewrite (or squash) some commits to clean up / reorganize related changes for the same feature. Or when you already committed something locally without realizing you were not on sync with the latest version of a remote branch you are working on and you don't wanna have it as a 1-single-commit branch that has to be merged.
Squashing with git merge --squash is also very situational.. ideally you wouldn't need it if your commits are not messy/tiny/redundant enough that combining them together makes it better.
I've tended to rebase before anything else. If I feel unsure about the massive interactive rebase fuckery I'm about to embark on, I just make another branch where I start. Then undoing it's fairly trivial.
Just use merge with informative PR titles, descriptions and linked work items. Reviewing history is then trivial and it has none of the pitfalls for less experienced devs.
I was trying to learn this again last week. I just play around with this stuff for fun.
If I want to consolidate all the commits into a a single message (to create a changelog sort of), which kind of merge do I use?
Another question: I'm torn between wanting to keep a complete history of my work, for my own benefit, and not wanting anyone to see how messy and crappy everything is. I've been trying to work in one branch then merge only when a task is "complete". But it's a bit confusing for me especially if I leave a project for a while then come back to it. Especially especially if submodules are involved. Is there some sort of convention about how to do this? Or am I thinking about it wrong?
You want to have commit history, not a commit fairy tale. Once you start rewriting history, it's not really history any more. The stuff people want to hide tends to be some of the most useful to someone looking through the history to find out how things became the way they are and what was going through the author's mind when it was written. If things are messy and crappy, it's better to know that rather than have it covered up.