As for Waterbenders, clearly it takes a VERY experienced Waterbender to be able to bend water inside someone's body. None of the benders we've seen in the show have had that skill, so it's probably on par with lightning with Firebending.

Which means Katara will probably learn it around the same time Zuko learns lightning.

Really, all of the elements are powerful. Fire and air are the easiest to have, since air is pretty much everywhere and you can create fire with your body heat. Water and earth typically require an outside source, except for extremely skilled Waterbenders who can use vapor.

Fire and water are dependant on outside influences, namely the solar and lunar cycles, and become useless during an eclipse of their patron. Air and earth appear to be equally useful all the time.

As was previously stated, each bending style has it's own strengths and weaknesses.?? Aang has been held captive because his arms were bound.?? An entire village of earth benders were held prisoner on a metal rig out at sea.?? There was a methane pocket at the Northern Air Temple that the Fire Nation army didn't know about.?? Had they, their bending would have ignited it.?? Katara was limited in the desert because of the limited supply of water available to her.?? To extreme extents, we know that a lunar or solar eclipse will prevent the water- and fire-bending, respectively.

Just like martial arts, there is no lame style.?? Each style has it's own strengths and weaknesses.??

I really like all of the bending styles. They are all useful in different ways, but they all have their weaknesses. If I had to choose my least favorite, it would probably be air. Sure they can fly, movie quickly, and move stuff, but I mean, how do they actually win a fight? By running away, disarming their opponent, knocking them down? It is really cool to be an airbender, but not my favorite.