What is that hostile mob you just can't wrap your mind around?

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budgieborb

When playing Survival, whether pro or noob, I'm curious to know: is there that mob that somehow gets you every time, when you least expect it whilst caving, or maybe its the Ender dragon you just can't quite defeat. Anyway, I'd like to know: does anybody have a "least favourite" archnemesis? If so please tell in comments and why! (Even mobs from add-ons/mods)

Caednahrgang

I play too much modded, but perhaps silverfish are my #1 slayer other than players.