Alsiel reminds me of overprotective Asian mothers who love their children too much. They usually handle all the chores so that their children can focus entirely on their academics and extracurricular activities. It's not my place to say whether that's an effective parenting method, but I'm under the impression that those who exercise this type of parenting didn't have the money or education they wanted when they grew up. Taking this into account, they provide what they can to their children so that maybe they can succeed where they couldn't.
In order to ensure that Maou can focus fully on winning the bread and figuring out how to return to Ente Isla, Alsiel readily assumes the role of house-husband. He takes care of the groceries, the laundry, the cooking, and other chores because they are far lesser in importance to their original goal: world domination.
Not only is he a superb homemaker, but he's also an excellent companion. Take for instance when Maou was at the hotpot, disturbing the customers by screaming in delight while his food was being cooked. Or the time when he took Maou shopping for clothes prior to his date with Chiho. I'm sure that had Alsiel not been there to look out for Maou both times, Maou would have a significantly smaller chance at maintaining his relationships with other humans. After all, they still need to navigate the social landscape of Tokyo, and Alsiel possesses the social adeptness to be his wingman.
On top of that, he uses his own time to take regular trips to the library to learn more about how to restore their powers. And if that's not enough, he also takes care of the bratty Lucifer. That's like being forced to adopt a spoiled kid! With their meager financial resources, Alsiel still manages to maintain the mental resilience to field Lucifer's complaints and laziness. I have no idea how he does it, but if that doesn't scream competence then I don't know what does.
Alsiel is the best waifu simply because he devotes all of himself to his lord. Compare him to the idealized waifu: one who listens to whatever you say, gently keeps you in line, and works hard all for your sake. Except in this case, the waifu should also be attractive; I'm not gay, so I'm not one to judge whether Alsiel gives you wet dreams.
Though if you think about it, his level of subservience most likely comes from the culture of demons from which he grew up. I would guess that back at Ente Isla, disobedience was met with punishment, and absolute loyalty to Satan was the norm. After all, Alsiel is one of Maou's generals for a good reason.
This also explains why this behavior is rarely seen on Earth: it has been a long time since anyone had ever pledged undying allegiance to another. Long gone is the era of royalty, and the time of the common man is now. Well, at least until Maou gets his next promotion.
