Well, I've never heard Demolition or Forbidden, so they're out.
Dedicated to Chaos I've listened to once in 2011, on that one website that let you play it before it was released (anyone remember it?). It was utter rubbish, had maybe couple decent songs, but most of it was pretty bad.
St. Anger isn't as bad as people make it. It has some glaring faults (production ofc, some songs go on too long, playing is a sometimes a tad sloppy), but it certainly has some real emotion. I don't mind the lack of guitar solos, their not necessary to metal music IMO (and Kirk has been playing the same solo from year 1992 anyway
). I hardly ever listen to it, but right now I'm having Shoot Me Again on the background and it is a pretty good song.
Virtual XI... well... I used to defend it in the past, but now I think it's a bottom three Iron Maiden album. Now looking back to it, it's pretty devoid of memorable songs. Como Estais Amigos is an exception though, one of their best album closers. The Clansman has its moments, but it drags on a bit too long. And I actually like The Angel and the Gambler and Don't Look in the Eyes of the Stranger, maybe because the overt repetition just becomes funny at some point.
Risk I used to like as a teenager, but haven't paid mind to it in recent years. It's a decent alt-rock record. Insomnia is an absolutely terrible song, but there's some hidden gems in there, like Prince of Darkness, I'll Be There and Wanderlust. I agree with Dave Mustaine when he said that if it wasn't released under name other than Megadeth more people would've liked it.