I can't stand people who sneeze overly loud... a simple achooo will do it. But noooooo I know peole who sneeze as if a tornado was in the room. I know a man who actually says "achooey uncle louie" when he sneezes.

Also people who pop gum, or chew gum like cows... chomp chomp pop slurp chomp. YUCK

Honorable mentions:
clicking pens
cracking knuckles