Largely because of Dawn of War 2, I favor Khorne.
The reasons (From that game's perspective at least) is pretty simple. All chaos armies have access to the Heretic (the basic unit. Largely expendable but fantastically useful at all points in the game) and the heretics can cease their direct offensive action in order to worship which conveys benefits to the immediate are based upon the champion you've chosen for the game. Khorne Heretics allow all infantry in the area to move faster and it allows for faster and more damaging attacks. Tzeench offers an area of invisibility and is largely useful for ambushes. Nurgle offers the ability to heal units more rapidly. Of the three, Khorne is the one that is the most useful. The heal is far to slow to be truly useful later in the game as firepower ramps dramtically and the advantage of the cloak can be countered by a single unit (not to mention the fact the worshiping heretics providing the cloak are not themselves cloaked, generally telling a decent player that there is probably a sizable force in the vicinity). By contrast, Khorne is always useful. It gets melee units into range quicker (reducing the potential damage they take on the approach), and it increases all damage output by a significant margin (nothing huge mind you, something like 5%, but that slim margin counts for a lot in a close fought game).
Beyond that, the follower of Khorne, the Chaos Lord, is a nigh unstoppable melee powerhouse who has an ability that is more or less a "win skirmish against poorly managed light infantry" button. With worship, his one flaw (he's god damn slow) is eliminated. Your staple infantry choice, the Chaos Space Marine squad, can go one of two routes: the mark of Khorne (they are fast high intensity and highly durable melee fighters that are even adequate against vehicles) or mark of Tzeench (they become superior ranged combatants and are upgraded with weapons that easily defeat heavy infantry armor). Often, from simple lack of numbers, you'll find that you can't really outgun your foe so the melee route is the way to go. When paired with worship, the beserkers move at hilarious speed and quickly arrive to bury chain axes in faces with wild abandon.
And, on the demon front, the two most generally useful T2 units are servants of khorne. The bloodletters do absurd damage and are great for breaking up formations or just drawing fire (they are incredibly flimsy but they can become immune to attacks (though they cannot themselves attack) for 10 seconds). The Bloodcrusher is all but immune to most standard weapons and does significant damage in melee. It is a bit vulnerable to anti-tank weapons but unless the enemy has a significant number of such things it generally isn't an issue. Better still, worship actually heals these guys rapidly at a significant distance so not only do they move faster, they do more damage and can take more in return!
By contrast, the Nurgle Demon is the Chaos super unit, the Great Unclean One. While it's hard to kill it also does absolutely MISERABLE damage (compared to other super units) and it moves at a speed that makes even the Chaos Lord look speedy.
From any other angle, I'd still say Khorne. Khorne is honest in his dealings. He is no master manipulator like Tzeench. He doesn't seduce with promises of pleasure and power like Slaneesh. He doesn't offer immortality like Nurgle. All he offers and all he demands is blood. Blood from the faithful. Blood from his foes. Blood from the bystanders. Khorne is unadulterated violence made manifest, destruction personified. He doesn't care if you fight for him or against him so long as you fight.