I haven't read much published AH, but I can feel a genuine stinker when I hear what goes on in the plot. The premise of
Stars and Stripes sounds mind-bogglingly absurd. The invasion of Long Island and the US by Imperial Germany in Conroy's
1901 must be SaS's equally demented cousin.
The
Draka series is also pretty dull and stupid with all the gratuituous hawking of its single gimmick : "ZOMG, teh bad guys win, dude !" Yay, hurray, I have never in my life heard of such a fascinating premise, thank you for opening my eyes, Stirling.
Then there's
The Year the Cloud Fell, in which the author himself apparently can't make up his mind on whether Mexico is 19th century independent Mexico or still the colony of New Spain and whether the American-Mexican war happened at all, since the western US is
virtually non-existant thanks to dinosaur-riding Indians and the continued existence of the Cretaceous inland sea. Yes, it's as retarded as it sounds. Dave Johnson has a hilarious review of it
here.
As for "most weird and yet underused to the point of complete irrelevance", I give you John Boyd's
The Last Starship From Earth and its POD of "Jesus randomly becoming an omnicidal crusader-type goon who leads giant armies of followers and fights with a crossbow, which leads to a darker version of Christianity with a crossbow for its symbol ruling the world for millenia". WTF premise with a boring sci-fi story tacked on, truth be told.
Also, any near-future military novel, where countries or factions, no matter their size, ideology, industrial capacity and logistics problems, conquer the world out of the blue (á la the Draka).
The
Thursday Next series would be horrible AH, but it's deliberately ludicrous for humourous purposes.
Oh, but the worst AH ever written has to be
this piece of AH, which is completely insane. I pity the fool who wrote it.