What's top of the list technically?

Oracle

Oracle is a giant mess, is the sense I got from trying to work with it in the past.

While I am loathe to recommend any Microsoft product, SQL Server is at least coherent, and has actual transactions like Oracle (and not just "checkpoints")

Aren't MSSQL and Oracle just doing redo logs, like MySQL?

What do you consider not actual transactions?

Postgresql keeps dead tuples in place until vacuumed, which seems optimized for rollbacks. But isn't so bad if you're inserting much more than deleting or updating.

Nestable transactions = “real transactions” to me, I guess.

I almost don’t see the point in non-nestable transactions

SQL standardizes SAVEPOINT for such nesting. Is that not sufficient? FWIW Postgres supports it.

Anybody that has ever used Oracle knows how nice it is. But very few people have that experience because their company isn't spending millions of dollars on the database server.

Yes, that was my point.

I get that working with Oracle and their army of lawyers is not cheap or pleasant but the db is still excellent.