Ahem
Multiplayer? Master ninja gear. The people who got to the world first would usually dry up all resources of claws and shoe spikes, of which one is impossible to get any other ways. And trading for it is hard because terraria multiplayers are very stingy on trades.
Singleplayer? Uhhh meowmere minecart mainly because moonlord is a bit difficult if your playing any difficulty but classic (or journey), and you have a lower chance of getting it.
pretty much all of these are easy to get with the right setups when on single player