Voted for Iron Skull and Gambler's Shield.
The reasons for this vote are pretty similar to what everyone else already said.
Iron Skull is a pure grind - you would have to lose and heal all your HP over eight times per level on average (not counting special levels - but even with all of them, it's still almost six times per level). No skill involved, no luck involved, it's simply an achievement for the sake of achievement.
Gambler's Shield is similar. Int1 at least is nearly a must there, cerberus boots or Enviroboots can help a lot here. But even then, it's one lever per level, and there are not that many games in which you can get this many levers generated at first.
The other medals either require some (serious) skill, or some luck, which I consider playable, and I'm fine with them.