The Element is LIGHT.

Focused light from a white dragon's mouth, will burn like a magnified beam from the Sun, and allows one that possesses that power to bend the light around them...like the Predator...a distorted cloak of invisibility. 

Of course...I could also be talking out of my Butt!

I don't know about Cynder's elements, because I've played this game with my cousin and I always played as Spyro.

Spyro:Fire(Because It Spreads From Enemy To Enemy and It uses Less mana)
Cynder:Wind(Because It fills up the Fury bar fast and it hurts Sevral Enemies at Once)

Best (in visual effects)
I have to say that I really like the look of Shadows and Electricity.
Especially when used together, lol.
Fire and Wind are nice too. And Ice.

Best (in strength)
Strongest element. Hmm.
Guess I would have to say Electricity for Spyro. And Fear or Poison for Cynder. Though Cynder's tornado attack gets hits up pretty quickly though.

I challenge you:

Use only Spyro's "crappy" earth element through the game and upgrade it all the way first.  I think you'll be surprised.  It uses little mana, and when used properly totally disables everyone in a decent-sized radius.

If I had to choose one for Spyro, I would stick with flame. It has great efficiency with mana use. I find I can use it for literally ages if I have to. Although it doesn't do as much damage as fear, its good if you just want to keep the opponent busy while the other player (If You have someone else playing) finishes them off.