Ooooh, so hard to choose! I like to think about the potential for elements in video games. Oh the possibilities for super-powered atrocity...
I mean, fires great for making people run around screaming, but there's always the chance the idiot will blunder into you or your allies and kill them too... and the AI is always stupid enough to make this really easy. On the other hand, I can see the potential of a fire-spell on a magical battlefield; can you imagine lobbing a fireball into a group of spearmen? WHEE, LOOK ET IM BURN!!!
Then there's ice, which doesn't give quite the same chaos bonus, but then again, it lets you freeze enemies and admire their horrified expressions as you use them as a garden ornament during parties. Still, once that wears off there's not so much you can do with ice compared to the endless hilarities that can ensue with fire.
While we're on the Final Fantasy X dynamic, lets go to lightning. And this, like fire, is fun. Do you shock an entire battalion of advancing forces because they were stupid enough to wear metal on a rainy day when there are puddles of soon to be super-heated steam lying everywhere? Use chain-lightning and watch your enemies skelentons glow as you rub your hands with glee? Bring the dead back to life and then kill them again for giggles? Right up there with fire on the chaos front, plus, may cause real fires!
Water... well, drowning large groups of enemies, washing them away with sudden flash-floods on a clear day, impaling them with high-pressure bursts... seriously, I remember an occasion in Kingdom Under Fire: Crusades, where you get to flood an orc camp. You get to wash away an entire army!
Wind... well, using hurricanes to pick up a large group of enemies and throw them into bins, walls and other enemies (and mess up their hair) also works wonders. One good trick would be to pick the enemy up with a hurricane and then position the hurricane next to a cliff; watching the enemy brain themselves off the cliff edge.
Nature is an oft overlooked one, and I have to admit, the ability to spawn tentacle monsters, feed your enemies to magically enchanted trees, make the grass drain the blood of your foes and cause terrible poisons and mutations are options I don't think have been fully explored by the gaming industry. I mean, what about poisons that could turn peoples bones into, say, white phosphorus, burning them from the outside in? Or spawning a pack of dire koalas on the approaching hordes face? The opportunities for hilarity are boundless, so I think this is my favourite.
Compared to the above, ground and rock, the power of the soil, seems rather dull, but there's potential. Make the ground literally open up and swallow the opposing force? check. Impaling your foes on stalactites? check. Cackling evilly as your foes get buffeted from side to side by constantly changing gradients, slamming into each other as they roll around? I could go on all day.
Overall though, making the world come to life and eat things appeals to me most though, followed shortly by FIRE!!! because it lets you burn stuff.