Should it be floating text like the match log in the bottom left, or should it be enclosed in some sort of shape?What kind of mood do you want the player to have?
Should it say something like "Target Destroyed," "Enemy Destroyed," etc. or should it be a big "+5" type thing showing your score change?
Or should it not be text based, and instead be an animation or sorts that signifies you made the kill?
(also I'm using a new method to keep things private. If you aren't signed in and don't have enough posts, then you shouldn't be able to see this image)
You could be silly and do something like "One cube down, many to go."
But I think "Cube Destroyed" would be kind of fun.