As far as the topic of the thread goes, I'm at a bit of a loss as far as one-hit wonder bands go. I'm definitely a "whole album" type of listener, so pretty much every band I listen to on a regular basis managed to put out at least one overall "good" album.
The closest thing to what the OP is asking for that I can think of is Exumer. Their first album is definitely solid second-tier German speed metal that distinguishes itself from its countrymen by importing influences from Exodus, Slayer, and other American bands, as well as a lot of "crossover" thrash mixed into longer epic song structures, but their second album is pretty generic and boring, apart from some admittedly decent S.O.D.-esque segments.
My favorite of their discography, however, is the demo they did right before breaking up at the end of the '80s. Every review I've read of it has been negative, mostly because the majority of listeners seem to go into the demo with a Municipal Waste Retro Thrash Pizza Ninja Turtles mentality and are then confronted with an exciting slab of progressive (as in progressive rock; there's definitely some King Crimson shit going on) technical speed metal with a more-than-adequate vocal performance from an early '80s NWOBHM band that never made it past the demo stage. If you like Voivod, Coroner, Deathrow, or even the first Meshuggah album (pre-AIDS), you owe it to yourself to check this demo
out. It's one of the few examples of speed metal legitimately leaving rock behind and approaching narrative song structures. It actually reminds me a lot of the "story-telling" approach of bands like Absu and Sabbat (the UK one), in execution rather than aesthetics (aesthetically, it's closest to Nothingface
I guess this is the band's "three-hit wonder," even though their first album is also worth a listen. I would recommend picking up the new digipack reissues of both albums, since the the reissue of the second contains the demo in the most pristine sound you're going to get.