Well everybody has a mob that they don't like personally i don't like Slimes and Magma-cubes at least not the big ones. I hate ghasts may cuz i hate jellyfishes and they kind of reminds me about each other.
And i don't like Endermans they are plain scary and are griefing my buildings and when they teleport at ya they make me fall of my chair!

And course my biggest fear the SQUID!

Do you have any mob you don't like let me hear.