Snipers. I hate hearing that "thwap", "thwap", "thwap" sound starting up. One seems to find me every time my defenses are on cool down, and I'm out in the open. My choices are to creep towards them and die just as I reach them, or creep away and die just as I reach something to block LOS. On the rare chance that I get to stealth up and go blast one off his perch I feel vindicated.