Go for shattered bonds. It's the best scenario from the base game by far. Challenging (but slightly easier than escape from innsmouth regardless of what the stars say), refreshing, interesting story, not too much monsters (cycle of eternity had way too much in my opinion), and doesn't overstay it's welcome (3 hours with 4 players).
In my opinion this is the best scenario, especially for people who like a challenge. It does not require prior knowledge of the scenario to have a chance of winning it (like in escape from innsmouth), but there still is a good possibility that you will not be able to beat it.
I absolutely agree with the above. This is the scenario that I have used and it is absolutely manageable for new players. I agree that it is by far the best scenario of the base game and that CoE is actually not that good, especially as an introductory scenario. The DLC scenario is also actually pretty good even for an introductory game.