IMO there are no weak album covers made by Riggs for Maiden. Everything spans from damned good to excellent. That being said:
11- Iron Maiden: Curiously enough this might be the most important cover of the list and yet I put it at the last position. Why is it so important? Since it is the character's inception it bears many of the traits future incarnations would be based upon and gave Maiden a "face". Nevertheless (and although it has tons of charisma and the light painting is really nice) it's a bit static and thus the one I consider to be the least gorgeous.
10- No Prayer For The Dying: As much as I prefer Eddie's eyes, the tombstone poem and the skeletal surroundings the 1998 re issue gave us (not only I like them I think these elements are better achieved in the reissue) IMO the grave robber is a must include on this cover. It gives so much dynamics to the whole damn scene. The dude's face is a bit too cartoonish but still cool and without him the art looks cool but has no impact. And once again the lamp's lighting... man. So another ultra strong cover... and yet on my bottom bracket.
09- A Real Dead One: Cool well drawn Eddie? Check! Lots of cool details and puns/ easter eggs? Check! Sense of dynamics? Check! Gorgeous chromatic combinations? Check! So yeah... it's a bit of a goofy idea (Eddie as a radio DJ) but other than this this cover rules.
08- A Real Live One: Now this dude looks aggressive! His face is almost caricature like but by some reason I like it... the massive teeth, the exaggerated size of the maw, the drool... And man, the pose is ultra dynamic. Not to mention how Riggs manages to mingle such vivid colors in a masterful fashion. Only down side is the electricity rays... they seem a bit off. Other than that great cover.
07- The Number Of The Beast: Ultra cool concept, lots of cool details, the painting's perspective is excellent and although the blueish sky always rubbed me the wrong way since 1998 we have the original black tone back and damn! It gives the landscape a whole new dimension when it comes to depth and wickedness. Superb.
06- Powerslave: So we're entering full masterpiece like territory and my god... just look at this thing! The attention to detail, the way Eddie can 100% dress the pharaoh's skin. Another cover full of funny easter eggs. Not to mention the grandeur and impact of the whole thing. Jaw dropping.
05- Piece Of Mind: This cover is something else. To begin with it's the first time Eddie is pictured bald and lobotomized. Then everything here is on point: lighting is superb (and that leads us to the door and other elements on the gate fold which are also top), the artwork as a whole gives an undisputed sense of motion, and Eddie's expression (and to a certain degree the entire scene) is at the same time endearing and disturbing. Flawless.
Then there are these four covers... just pick one because they're all on a really unique level.
Killers: Talking about disturbing and capturing a moment? Killers takes it to a whole new level. Although being the sophomore cover, it's already filled with details on the background but what really captures everyone's attention is Eddie's sadistic grin as he turns to you with a "you're next" expression while his last victim is still grabbing his shirt. However the aspect I think Killers beats every single Maiden cover - the only one I recall coming near is Fear Of The Dark or Live After Death - is the lighting: not only the sky and clouds are perfect (but regarding that Riggs as several covers where he harnesses those motives with the same level of skill) but the way the lamp's light interacts with Eddie... Woooooooow!!!! Mind blowing!
Seventh Son Of A Seventh Son: if you like Riggs and Dali as much as I do then this cover can do no wrong. The icy environment is flawless, the easter eggs are top notch, the surreal elements are really well thought (the floating lamps, the apple on Eddie's chest and his burning opened skull, his body coming apart piece by piece or the way the water bends on the back cover). Then there's Eddie's expression and the elephant on the room: the fetus. Plus the chromatic harmony being totally disrupted by Eddie's figure simply belongs to a class where reserved to a small elite. Pure art.
Somewhere In Time: Everything here is over the top. Easily the most detailed and easter egg filled, this cover is a feast for every Maiden fan. even the Blade Runner like motive, the architecture and the clouds and lightnings on the back cover are flawless. Then there's cyborg Eddie himself (easily my favorite incarnation of the mascot) and the whole dramatic action as he just shot another poor cyborg to oblivion. So good it's ridiculous!
Live After Death: So let's see... lots of well penned easter eggs and details, a realistic sense of motion regarding the storm, once again the cloudy moonlit sky is godlike but man... the dynamics of Eddie arising from his grave as lightning (here they are superbly drawn) hits his lobotomy plate, the burning underground and how the light affects Eddie and everything in its proximity, the floating flames, the lightning passing through his shackles. I swear... this is sick. Just so perfectly done it's sick, period.