Thank you,
Gohara, for another detailed list.
I agree that One Piece sometimes (well, most of the times) has too many characters on covers, but I really love some of them. The chronological list would be:
~ Volume 1 [it really is iconic]
~ Vol. 6 [the version I have doesn't have a black background, but a very cool looking dark gray; out of 9 volumes I actually have, this is my favourite]
~ Vol. 9 [the composition is very neat with Nami as a frontal figure and others in nicely drawn panes behind]
~ Vol. 13 [the same from above applies here]
~ Vol. 18 [again, the same as before applies here; plus there's Ace on the cover
]
~ Vol. 21 [nicely arranged CP9]
~ Vol. 22 [nicely arranged Starwats]
~ Vol. 26 [not too many characters, well drawn, awesome atmosphere]
~ Vol. 28 [this one has a lot of characters, but I really like how all green it is]
~ Vol. 30 [Enel for the win]
~ Vol. 32 [nice composition; simple but good]
~ Vol. 37 [I like how highlighted Franky and Iceburg are]
~ Vol. 41 [Sogeking]
~ Vol. 44 [Robin, Rob Lucci and Luffy; what's not to like!]
~ Vol. 46 [Brooooook]
~ Vol. 48 [OARS]
~ Vol. 50 [awesome composition; and now I think Oda's best covers are those with separate panels]
~ Vol. 57 [Whitebeard]
~ Vol. 58 [Admirals + panels again]
~ Vol. 59 [Ace, iconic]
~ Vol. 61 [a new start, iconic]
~ Vol. 69 [this one has too many characters, but it actually works because Law and Zoro are the biggest there]
~ Vol. 76 [Oda copied Kubo's current cover style and placed it in just one panel, hehe]
~ Vol. 77 [again, there's a lot of characters, but it actually works a lot; I like the dark background]
~ Vol. 78 [the same as above]