The Starter Set and The Rise of Tiamat covers are fantastic. They're simply perfect. They're well-drawn with a bright palette and dynamic images, although the images are a poorly cropped so you lose some details in the lower half of the Starter Set and a good chunk of Tiamat gets cut.
The Monster Manual is decent. The beholder is a nice choice for the cover, being iconic to D&D and only D&D (excluding the dozens of other games that rip it off). There's obviously some stuff going on off panel, but the cropping is less obvious.
The Dungeon Master's Guide cover is adequate. I imagine the idea is to have a mastermind style villain on the cover, and the lich certainly fits that bill. But having him be a physical threat to people just casts him into the role of "monster". This could easily have been the cover to the MM. If the lich was in his lair, perhaps looking commanding and in control it might convey some "mastermind" element. Or if he was directing monsters rather than wading into the fight himself.
The cover suffers from the need to provide an action shot. And the image feels poorly cropped.
(Looking at the cover again - having typed *just* the above - I can see the lich raising a fallen warrior not sucking his life out like I had previously assumed. That would be a better image made unfortunate by the choice of a not obviously zombified figure.)
Hoard of the Dragon Queen is alright. There's nothing special going on there. The colouring is poor, making what is presumably a white dragon look rather purple. There's not a lot of colour in this one so it seems rather bland and, paired with the MM and DMG, makes the books seem rather limited and dark in hue.
The Player's Handbook is terrible. A fire giant is a poor choice for the cover, taking up too much space relative to the heroes. You can only really see one adventurer, which doesn't convey the "team game" aspect nor the focus on the heroes. And while using a "Name" giant is a neat idea, the white dragon skin is distracting and seems contradictory for a fire based creature. The position of the adventurer's leg looks weird when paired with the giant's shirt; until I saw the larger sized image I wasn't sure what was going on. And the cropping of this image is particularly bad.
But at least it has a different colour palette.