I thought 2300AD's covers were frequently rather awful (a shame to the game, actually), so I do hope QLI can produce something better.
If I have to select three particularly silly ones, I'd take
-Nyotekundu. Look at the planet and the ring system. What does the ring orbit, and why does it look like that? Terrible, and in close competition with "Invasion" a good choice for Worst 2300AD Cover.
-Colonial Atlas. Most strikingly, the terminator of the planet. Hint to would-be 2320 artists, take a brief look at photos of real planets before doing such artwork.
-Aurore Sourcebook. Look at the horrid black-white moons in the sky. It could maybe be partially defended with some intricate assumptions about a near-Tithonian eclipse (otherwise why would they look black?) and a distant focus combined with some very weird lightning, but still...
I kind of liked the cover to "Operation Overlord", though. And "Ranger" had a bit of a naivistic touch which also was kind of cool. "Ships of the French Arm" is kind of nice too, it is at least cleaner than the other space covers.