Rant--you can skip this.
I have a large collection of Discworld editions. I would buy the US hard cover edition to read it ASAP, the US SF book club edition [sometimes easier to get than the standard one, and often better covers], the UK hc edition to see the differences in the text -- often very large, though that ended when they started printing them at the same time [but still, some changes were made]. Then the paperback editions [both UK and US , when available] for ease of carrying, and newer ones for the new covers. I have an early pb of TCoM with a non-Kirby, non-Kidby, non-book club cover picture.
Until I figured out that Kirby was hiding a different picture in his weird covers, I hated them. I still prefer the early Kidby ones before he forgot how to draw. I actually liked the US "simple icon" covers-- they were better than some of the hb covers, IMO. At least they were recognizable from a distance and the icon had something to do with the story, and they were brightly colored so you could find them inside a bag. [Why do they make bags with stygian black interiors? and then make everything you might carry covered in black?]
Even the black covers with a photographed object were trying to tell you something about the book, even though their stated purpose was to let people read without being attacked emotionally by family and colleagues who sneered at anything with a fantasy. cover--or, in some cases, anything that wasn't depressing.
So the worst cover IMO was the one with Vimes walking through a mirror and losing his clothes, and that was mostly because Sir pTerry hated Alice in Wonderland. Also it was a spoiler.