Best:
Really getting into Rust. It has taught me so many things.
1. Null is evil
2. Sum types are amazing
3. Compiler can actually have good error output
4. Multi threading is actually really scary if you don't have a compiler to back you up

Worst:
I had to deal with SSIS. It has also taught me many things:
1. No matter how 'mature' a product is, it can be awful. Simply dump a random error code, the user can figure out what went wrong, no need for good error messages.
2. The modern concept of the database is crap. It's a gigantic global state that is used by everyone and owned by no one.
3. Don't use tools that aren't made to be used with version control.
4. Even when you tell your team that it's bad, you will be ignored.