So many great album covers, but I think Rubber Soul stands out for me. I like Beatles For Sale too, but I think the back cover is even better than the front.
I think Sgt Pepper just takes the prize ahead of Abbey Road as most iconic... just because it was less copied than Abbey Road (which is so simple and therefore easy to parody) doesn't make it less iconic. Pepper was so perfect for its time, and bursting with elaborate detail and colour.
Sorry about this next bit but...
Revolver has to be by far the worst Beatles album cover for me. I know many people find it very arty and cool, but I actually find it quite drab and almost depressing. Different, yes... but flat, amateurishly doodled (IMO) and scrappily pasted together with no colour anywhere (b&w worked brilliantly on With The Beatles... but this was meant to be psychedelic '66 for goodness' sake!). For me it's the only (relative) duffer in an otherwise enchanting string of sleeves.