Barring Death By Carrion, DK. He's the most efficient creeper out of the 3 heroes. But I usually go rapid triple hero. Despite what some people may think, 3 heroes is really powerful. With UD triple hero, you have the most powerful hero sniping team in the game with Coil+Carrion+Nova. Or, alternately, you can use Coil+Nova for hero sniping and just abuse Sleep as much as possible. This is a bit more secure against times when you're up against multiple heroes or you just want to keep the enemy hero almost completely out of the picture. Plus you get the dual auras, and the ability to heal all the heroes. The chances of getting an Ultimate when you go triple hero is usually quite slim, but I don't go for Ultimates. Games should be decided before Ultimates hit the table, at least concerning UD IMHO.
If I am going for Death By Carrion, then I'll actually use the DL first, followed by the Lich. Provided you can harass well enough and abuse potions enough, you will be able to take on any ground army with Carrion+Nova and Frenzied Ghouls. I don't usually do this though, simply because I feel insecure without the DK.