Okay. For Red Skull, consider a pair of standard black legs.
For Spider-Man and Cap, I use the blue legs that the regular Spidey and Cap use.
Alternatively, you could use the dual molded Spider-Man legs from Sandman/Rhino Supervillain team up set.
I haven't tried this, but the Lego Minecraft Jungle Tree House has a pair of plain metallic silver legs that would be good on Ultron.
For Hulk and Green Goblin, Dark Purple Legs from The Joker Minifigure in the Joker Steam Roller Set should do the trick.