Hecarim does decent damage and gets in and out pretty easily doing psychological damage to their team.
That being said, a Skarner picks the biggest threat and with a Flash+Ult combo will immediately remove the enemy carry that is the biggest threat. This is a no-brainer you can't fail kind of ult.
I would personally pick Skarner with a Sheen his damage is respectable and he has a very good running/chasing ability with a very powerful stupid-proof ult. --- "In the beginning the universe was created. This has made a lot of people very angry and has been widely regarded as a bad move."
Hecarim. Versitile, and does things the others cant (he can gank harder, has insane team fight presence, can escape better than lee sin if there are no walls [with walls lee sin has teh edge with his little leaps], and is will be strong without money, or if fed, snowballs totally out of control). --- These idiots won't understand hyperinflation until it takes $100 in food stamps to trade for a bottle of gin.
The jungle's changing very soon, it's impossible to say without knowing what the new jungle will be like. --- "We all wear masks, and the time comes when we cannot remove them without removing some of our own skin." - Andre Berthiaume, Contretemps