While not my favorire AH so far (that's still Years of Rice and Salt), I'd posit Robert Harris' Fatherland is the perfect AH novel, and, if you're a layman, the only AH you'll ever need to read.
As the good High Concept story that it is, the gist of it is easily recognizable and digestable to your average reader of detective novels, thrillers, bestsellers, etc., that is a layman with no idea of what a counterfactual is, while also being attractive to your garden variety history or military buff. Thanks to history and pop culture, there's few settings as common and familiar as Third Reich, few bad guys as ubiquous as the Nazis. Sure, keeping the same system, the same organizations, the same names, the same chieftains in place over a 20 year period might stretch credibility to pedants like us, but it is rewarding to the average history fan to recognize those names and have a clue as to what is going on.
The Holocaust might be an obvious plot to uncover, and maybe the art plunder might have been interesting enough on its own in the hands of a great writer, but it might have been a disservice to make a novel about the Nazis Victorious and not center it around their biggest crime. And, once again, for the layman, there's no better way to keep the stakes high: uncover the horrendous thruth of the Final Solution or the Nazis win the Cold War.
The main characters are also perfectly built to exist within the world showcased by the story while also retaining the reader's sympathy: A middle aged workaholic police officer with a broken marriage, living in a crummy apartment, what could be more relatable to western audiences living under the aegis of American mass media? What did he do during the war? Stuck in a sub, not bombing cities or fighting our boys (or shooting partisans in The East), far away from the most disgusting of Nazi crimes, so plausible deniability for not knowing, while giving him Stranger in a Strange Land points. SS membership? Mandatory and he hates it, thank you very much. Party membership? Doesn't have it, so he's a cool rebel.
Charlie Maguire is also perfectly built, right down to her tomboyish name and haircut. Beautuful, feminine and petite, she's also fiercely independent, strongheaded and an intrepid reporter who makes all the first moves in her tryst with March, even though she's half his age and he's a Sturmbannfuhrer in Nazi Berlin, so we never feel either a power imbalance or that she's a "slut/floozy." So she can be a fantasy for the average male reader, particularly the downtrotten middle aged ones who would see themselves in March, while deflecting criticism by not being a shallow love interest and instead being a, perhaps stereotypical, strong female character, all while being MeeToo proof 30 years ago.
Of the villains, the main one, Odilo Globocnik, does stand out as a truly sinister, odious presence. A creepy, sadistic, vain, narcissitic brute with zero morality or scruples, he perfectly encapsulates the true nature of the system he represents, even better than the superficially charming and grandfatherly but utterly self-serving Nebe or the decreptit and unseen Hitler. On the other hand, Globus is 60 at the time of the plot, so some scenes are a bit head-scratching, even if we make allowances for very well-kept sixty year olds who exercise on the daily.