DK first, no questions asked. As I have lectured over and over about on the old posts (poor x42 had to bear the brunt of my lectures, sorry dude ^^;;

, he's just too good to pass up. Getting him to a higher level first affords you a powerful tanking medic, and a lethal hero sniper. In addition, he is pretty much the most efficient creeper in the game.
As for other heroes... Well, though I'm not much of a TFT player, I'd say DK, CL, Lich for powerful crowd control and hero killing, but it's all up to you. I've noticed a dual aura CL is pretty hard to take down. Either way, I usually like having DK and CL in a single combo, since they're both good tanks, though one is more dependent on the other than the other is (wonder who that is...).