The Thin Man (1934): Of course, every fool knows that what makes the Thin Man movies such endurable classics isn't the mystery part of their formula (though there sure is a mystery, murder and crime) but the interplay between Myrna Loy and William Powell who were clearly born to play this sort of part in exactly this type of movie. Even 80 years later, it's still a joy to watch them playing one of the most matter of factly romantic couples, having what looks like the time of their lives, throwing each other one great line of dialogue after the next.
After The Thin Man (1936): The second time around, Nick and Nora's charms have a more difficult time in a film that sometimes seems to go out of its way to put stuff on screen that frankly isn't as interesting as its two core characters, needlessly prolonging a story that would work better if it always were as sharp and snappy as it becomes when its stars are actually on screen. Worst among the unnecessary additions are James Stewart (in what might be one of his worst performances) and Elissa Landi being "dramatic", and a "funny" subplot about the troubles of a dog marriage. The film also suffers a bit from the fact that our heroes' flirting needs to be quite a bit less risqué than before, for the good pre-code times are over, and even quipping alcoholics now have to be more responsible. Still, whenever Powell and Loy are together, the film regains the magic of the first part, there's just a lot of feet-dragging and filler surrounding them.
Another Thin Man (1939): The third film finds the series on surer footing again; while the puritanical streak of code filmmaking isn't going away - though the film skirts that line more than once - and there's a sad tendency to make the alcoholic partying couple of the first film more responsible, Another Thin Man knows much better what to do with the things it still is allowed to do. Returning director W.S. Van Dyke also finds an actual interest in the mystery plot this time around, actually connecting Nora and Nick with it in a way that doesn't leaves it as filler coming between the audience and the good stuff but an intrinsic part of the film that's worthwhile in its own right. I also found myself much less annoyed by Asta's dog shenanigans this time around; the mutt still feels like the smuggest show-off of a dog imaginable, but at least her scenes stay short and actually belong into the movie I was watching.