Making a comeback is always a rush, especially when the other team was doing a 'little' taunting before you do it.
Being in a large (3v3 or 4v4) fight, getting the first kill, then proceeding to roll the other team is nice.
The single best moment I personally have ever had was a VERY close total PUG match that my side ended up winning. As the citadel collapsed and we all issued our gg's, one opponent, Lokida13, said 'F*** You Sedna' This made me very happy
(guess he didn't care for the silence/pounce interrupts)