I'm gonna go with the Darkness (aka the unconfirmed fifth race). I chose the Cabal in the poll, because they are a military regiment. Their way of philosophy is war. They're enemies which are going to fight until the last of them is no more. The Fallen are scavengers, which hate humanity wholeheartedly but don't seem to rely on tactics a lot. They're persistant but not as dangerous as the others. The Vex can travel in time but since they are robots they can't deal with unexpected events and have yet to analyze new situations before they are able to take action - if I'm proven wrong on this, then they are the major danger. Hive; high quantity but little to no tactics. In a gun fight they can overwhelm their enemies with mass, but in war you just have to drop a bomb and take on the remaining Hive.

The fallen seem most original. (am I mistaken?) It seems they will be most satisfying to kill.