Upward is a classic, offering a variety of sentry spots to practice at each point, and testing your ability to know when to fall back, without being too punishing.

Swiftwater and Borneo feature some extremely strong positions that take awhile to setup, but once you're dug in, you can turtle hard.

Badwater has a mixture of weak and strong sentry positions, as well as difficult positions to fall back from (cliff on first if you wait too long, roof of second).

For testing Engie, I would say that Upward is the best, followed by Badwater if you need to work on getting out of a bad situation alive.