The Manowar cover never bothered me, but I like that album. Though, I do recognize the cheeze factor to be sure. The Riot, well, I've always thought that was odd. Trilogy bites (thought so back in the 80s too - the cover that is). And I always thought Bonded by Blood was ridiculous....just poorly drawn really, not the concept altogether. Violence and Force is silly, but not terrible.
As for greats......they left off a bunch I like. Two that come to mind immediately are Melissa and To Mega Therion. I'd include Monotheist now too. A very great and disturbing cover. I'd also include, rather high on the list, Born Again. It still disturbs me, but is a great, great cover. And, pretty much all the Motorhead covers are great.....but Overkill was a fine choice. I'd probably have chosen Another Perfect Day or Orgasmatron. Also, Sabbath Bloody Sabbath is truly great. A perfect metal album cover. Oh, and for me personally, I'd have chosen Number of the Beast over Killers. Soundmaster, you're right about the quality of Maiden's covers dropping off. This phenomenon reached its height with DOD. They are kind of asleep at the wheel on presentation these days, music aside.