Headshot on a moving target from a decent distance in BF3... It's just epic, when you let 1 shot fly and it hits the target right in the dish. I like the SVD scope for this, seems to make the most sense when trying to judge drop-off adjustment.
Ohh, or hiding in some boxes or broken scenery and watching the enemy run right past, then you get them in the back - it's most effective when you do that to about 5 or 6 guys before they realise where you are.
Also, killing anyone in a helicopter in BF3, then watching it plummet

.