For pure leech-burning destructive force, nothing beats fire. But as stated above, the other elements offer various forms of flexibility that any one element would have trouble with. It's hard to ground magic with fire, but water can do it. It's hard to knock people down (without burning them) with fire, but earth or wind could do it. If you want to destroy a door completely (along with part of the wall and perhaps the whole building if the fire crews take long enough, fire's your tool of choice -- but spirit could slam open the door with far less collateral damage. It's hard to imagine any reasonable defense based purely in fire. A wall of fire might hold something back through intimidation, but even a determined opponent willing to risk a few burns might have trouble getting through a wall of rock or even force. For more of a Warlock bent, you can melt heads with fire, but short of that mind control is not a strong point for fire.
