I count GotS & the Hawaiian duology among his best works, along with "How Few Remain."

Turtledove's problem is that he really can't think too far afield. He can think up an alternative to an historical event, but he can't carry on beyond the initial premise. The series that grew out of "How Few Remain" initially presented a number of cultural differences in the alternate timeline, but some of them faded away and were forgotten and others were deliberately reversed when Turtledove gave up on originality and went back to telling the real world history with different names, which, when you get right down to it, is his only real strength.

Almost everything Turtledove writes is basically a real historical event dressed up in different clothes. TL-191, despite its promising start, degenerated into the Nazi-Soviet conflict of WW2, fought between American nations. A fantasy trilogy was nothing more than the western campaigns of the American Civil War, that was more of an in-joke for history buffs, with historical figures simply given different names (Gen James Longstreet = Earl James of Broadpath; Daniel Harvey Hill = Baron Dan of Rabbit Hill; Jefferson Davis & Abraham Lincoln = King Geofram & King Avram). The Darkness Series, is World War Two in a fantasy universe with magic replacing technology (i.e. Nazi & Soviet atrocities are committed because human sacrifice empowers magic). In both series, Turtledove simply turns the map around (so the Germans push WEST into Russia, and the Russians falter in the harsh desert climate of Finland) and flip the races, so the alternate civil war series is about swarthy people battling over whether or not the fair-skinned, blonde-haired serfs will be freed and the alternate WW2 series features a red-haired master race oppressing & sending the blonde-haired, blue-eyed enemy race to concentration camps. Or the fair-haired island empire with a strict code of honor launching naval attacks at the powerful, wealthy US analogue, populated by swarthy, slanty-eyed people.

His older fantasy novels might not be as immediately recognizable, but they are, for all intents and purposes, set in the Byzantine Empire, and retellings of the stories of Byzantine historical figures.

This characteristic might be better concealed in his 'alternate history' books, but as I noted, TL-191 reproduces the Nazi-Soviet conflict in America. Another book set in a victorious Nazi Germany retells the fall of the Soviet Union, and a book about a Nazi resistance movement against the Allied post-war occupiers is a very thinly disguised analogy of the Iraq occupation. In the first and last examples, he maintains the parallels even when they don't make sense. He previously establishes the USA as technologically superior to the CSA, but then reverses that comparison in order to make their conflict fit perfectly with real world history, because the USA in his world is playing the part of the technically INFERIOR USSR in World War 2, and the agrarian Confederates have been assigned the military role of Germany, an historic industrial power. The reasons for the Soviet Union's fall don't apply to his alternate Nazi Germany. The circumstances of the Iraq guerrilla movement don't fit those of a post-war Germany, but Turtledove makes them the same, to the point where he is even forcing plot points to make controversial arguments about the current crisis, and with that conflict thus far unresolved, he has no basis on which to write an ending.

Finally, any long-suffering Turtledove fan will tell you to beware of this: cumbersome, repetitive dialogue. And the same jokes being told to make characters appear witty. "If you were half as clever as you think you are, you'd be twice as clever as you really are..."is a favorite, that sometimes even crops up more than once in the same series.

As it is, a lot of the problems can, I think, be attributed to his output, and current books are not so weighed down, mostly, I think, because he has reduced his workload by concluding a number of series. Also, Turtledove seems to write his best stuff when he is circumscribed by some parameters that prevent him letting his (non-existent) imagination take over the story. Guns of the South and How Few Remain were excellent, and featured historical protagonists and PoV characters. He writes a lot better when he is trying to duplicate the thought process, or compose dialogue or writings for Robert E Lee or Frederick Douglass or Samuel Clemens than Sam Yeager or Chester Martin or Flora Hamburger-Blackford (if you think Egwene's Mary-Sue debate victories in tGS & ToM were appalling instances of Plot-Mandated-Enemy-Stupidity, you haven't seen nothing until you see how Flora wins arguments and sways crowds with speeches). Other times, he seems to do better if he sticks with a particular notion and limits his scope. Some of his short stories are quite good, but long series tend to ran away from him. There is also his habit of sneaking in little shout-outs or concealing characters in his series. In the alternate timeline of the TL-191 series, Adolf Hitler never comes to power in Germany for various historical reasons, but he actually makes an unheralded cameo appearance as an unnamed background character. In another instance, an office is swept for bugs by an electronic security team consisting of Bob Woodward, Carl Bernstein, and Richard Nixon. A young naval officer on leave is suggested to be Jimmy Carter. There are similar little cameos and shout-outs throughout his books which make them fun for a history buff, but the sheer number of them can get tiresome and sometimes seem too cute and excessive when the story is lagging. He can also be rather obnoxiously and hypocritically pro-Jewish, but that's also probably a matter of preference or cultural issues that he might not even be aware of what he is doing.