If you're going for a REALLY powerful attack, having it affect the entire screen in some way (however small) is good. Screen shaking is the obvious one, or some sort of radial blur. One of my most powerful-feeling attacks is Typhoon, which is a flying uppercut. When it hits, the screen pauses for a few frames (and shakes afterward), there's a flash of light, the screen blurs away from the point of impact, there's a fireball explosion, and there's a sharp sound effect. I have a picture of it here
but that doesn't really show much - the emphasis is on all of that happening very quickly, and then the enemy flies across the stage, hits a wall, and explodes. It also breaks through the ceiling if there's one above you, so it really feels like nothing can stop you when you have it.
Even though it's actually not the best weapon, it FEELS like it, just because of that giant impact. It's similar to what was being said about Smash Bros before.
Special note also goes to Mega Man Zero (and ZX), where killing an enemy with the beam saber slices them in half, and killing an enemy with a charged buster shot causes their limbs to fly off. Even the bosses.