Realistically, the answer is whatever game you understand well enough to be able to comfortably and comprehensively teach. Your ability to teach the game matters the most.
Agricola wasn't on my short list of entry-level worker placement games yet I and other folks I've known have had great success with it as someone's first worker placement game, even when not using the simpler family-level rules. And Agricola has a reputation for not being an entry-level game. Perhaps falsely. After all, the theme helps make the game accessible since everyone knows what you do on a farm.
But, this mental exercise is about picking games that are what games we think would be easy for folks to pick up. My shortlist ended up being Lords of Waterdeep, Sticky Fingers and Stone Age. Since it's popular mechanic, I am going to bet there are many good candidates that I have never played or even heard of.
Sticky Fingers is one of the simpler worker placement games I've played and I do like it quite a bit. But it doesn't involve engine building per se, which I think is an important part of the worker placement experience. Lords of Waterdeep, a game I've come to appreciate the more I play it, has the opposite problem. The engine building is more subtle and complex than I thought at first.
Stone Age ended up being the game I thought would be the best game to introduce worker placement to people who think that's another name for temp agencies. It is relatively simple but very dynamic. And, while the engine building just invokes adding people, tools and agriculture to your tribe, it's still engine building.
It doesn't hurt that it's a game that I still enjoy playing. My personal introduction to worker placement was Pillars of the Earth, a game that I still think is a strong design but I grew tired enough of that I eventually got rid of it.
Of course, this is all just a mental exercise. When it comes to actually doing introducing people to worker placement, what really matters is what games you actually have access to and enjoy enough that teaching them will be fun for everyone.