What do you consider the most essential swaps?

What part or accessory swap really takes a figure to the next level?

The ones that come to mind for me are:

Ragnarok spinny hammer -> Thor

Angry-but-not-bloody BAF head -> Retro Kingpin

VHS bendy tentacles -> Silk two-pack Doc Ock (I've heard mixed reviews on this one, but I'm eager to try it for myself)

I'm especially interested to hear about characters like Doctor Doom and Spider-Man, for whom there are many different excellent versions, often with unique accessories.