You shouldn't judge a book by its cover but you can judge a cover by its art. I fully intend to list my favourite Doctor Who book covers but first I have to get out the way those I dislike the most. Some are boring while others are misconceived...
Image of the Fendahl
I think a lot of the target books have art of questionable quality, but this one stands out as just being hard on the eyes. It's just a bit nasty with those very rich colours that clash, that yellow writing on the intense blue. The rest isn't up to much either, the Doctor posed with a Fendahl behing him, I'm not sure if they are just placed together or whether it's supposed to depict an actual scene in the book. I find it pretty ugly, yuck.
I know this is a book cover that has received criticism before. The New Adventures had mixed standards of art too but many were pretty good. Alister Pearson covers are generally among the best, he did many reprints of Target books for Virgin and most of the Virgin Missing Adventures range and I don't list any of that range among my least favourites because they are at least competent or very good. The New Adventures range seems a bit more hit and miss. Towards the end of the range they went for smaller pictures on the covers with a sort of swirly shape above and below.
Back to the cover of Set Piece, it's just awkward. Ace is depicted in the style of a fantasy novel, sword wielding and minimal clothing. Her face is rather expressionless, the pose doesn't seem right to my eye and she lacks a belly button too.
Thinking about Kate Orman books, The Left-Handed Hummingbird is rather odd too. The art is ok but just what is the Doctor doing?
The Janus Conjunction
I've always been of the mind that I prefer painted and drawn art over photoshop jobs on books, This is why I still appreciate the efforts of the Virgin NAs over many of the BBC books. It's too easy and formulaic to find a stock photo of the Doctor and a monster, put some filters and effects and call it a day.
There are numerous covers just as the above, particularly early on in the BBC range, that are pretty basic. The advantage of the photoshop covers is that they are safe and almost always produce a competent result that is not offensively bad, rarely do they look awful, but equally there are very few that are outstanding. Some of the Virgin books are superb, Blood Heat and The Dark Path come to mind.
But the Janus Conjunction stands out as one of the most boring covers put on any Doctor Who book. There is almost no feature of credit here. It's trying to depict the eclipse that the planet in the novel is under, but you wouldn't know that just picking it up. What we have is something that looks like water going into a black void that fills the cover, as though someone forgot to put something in there.
Arc of Infinity
Dear God this cover is awful. Apparently, or so I've heard elsewhere, Peter Davison wasn't keen on the illustrated covers around this time because of the dubious quality. But that doesn't excuse the alternative which must have been the cheapest covers produced for any Doctor Who book. Arc of Infinity stands out as a total failure. Firstly it appears to have been created by someone cutting up a copy of their Radio Times and sticking the pictures on a repulsive yellow background. Secondly, it depicts the reveal of the traitor which should be a surprise. Wouldn't it be better to have Omega be on the cover?
This was a trend around this period. The Visitation was the first book with a photo-cover but as it was one of the first publicity photos it could be excused, further the original art was apparently quite poor. After this came a number of photo covers and a run of the dullest covers of Doctor Who books published. Mawdryn Undead merely has the Doctor standing in his Tardis with a beige background. Terminus goes for the Arc of Infinity approach by cutting a couple of photos out and pasting them overlapping regardless of perspective. Earthshock just uses a photo on the bottom half and just gives up on the top half making it plain blue. It also has the Doctor posing with a gun, which does happen in the story but ends up making this have a very un-Doctor Who-like cover.