Good lord, I do think that Liefeld fan is high....but best cover ever? I wouldn't even rank it in my top 1000. It's a very, VERY good illustration, but not a good comic book cover IMHO.
it depends what you're after, Batman 225 or 235 are my favourite for telling a story but Killing Joke is so memorable that i think it surpasses them for what a cover is supposed to do (i.e. being attracted to/buying the book)
Even then, the only reason it draws someone in is because Bolland is such a good illustrator. The image itself isn't particularly interesting from a design standpoint.
it's a very simple cover, sticks with you when you see it
this is probabily my favourite
or this
but it's not as memorable