Mine has to be a sparrowhawk. I was messing about taking photos of a small group of starlings going overhead, when they wheeled around at came back past me. I carried on photographing them and through the viewfinder realized why they'd changed direction. A sparrowhawk was eyeing them up!
The starlings escaped unscathed this time, but the sparrowhawk flies over fairly regularly, usually just scouting and avoiding the corvids.