As mentioned the later US releases that used the original Japanese art were
best, but I've always liked the cover to the original Dungeon Explorer with
the weird abstraction going on in it.
The absolute worst cover imo is Legendary Axe II. Two generic barbarians
holding axes, bad perspective and dull color. In the game your character and
enemies have a "Aztec" look to them and axe isn't even the main weapon, you
start with a sword.
Here's some food for thought. The beautiful cover to Ys 1 and 2. If I'm not
mistaken this the only turbo game prior to the rearrangement of the company
that used the original artwork. I wonder how come it was spared? Ys III had
a great Japanese cover and they redid that one (badly) for the US market. Can
you imagine a Ys 1 and 2 cover done like other turbo games (shiver). *-neo