Easiest kill?
You'd be surprised... when people spawn right in front of you, sometimes you are shocked so much that they kill you instead of the other way around. Happened to me before, I am ashamed to admit

But I would say the easiest kill is when someone is plant the bomb in S&D, or defusing, and you stand behind them and kill them. They are so defenseless, with a big briefcase in their face. Even if they realized you were there, well it takes a second or so to stop defusing/planting and get up.
Even last stand people can turn around fast and knife a t-bagger, but when you are planting or defusing, you are helpless.