What is survivability based on?
The human wizard is the most survivable character in our group. He has gone down maybe once in 5 levels, where as everyone else goes down a time or two per level. And he never even runs low on healing surges. The elf archer ranger is probably the next most survivable. As a leader, I rarely have to worry about healing the two of them. The ranger does have to move around a bit more to quarry the right targets, which occasionally puts him in danger, but that's why he has hit points and healing surges. He can usually get out of trouble (or I can get him out when needed).
In the days of broken battleragers, our dragonborn battlerager was on the same boat, I never had to worry about healing him. But since the fix, he does go down occasionally, and I do have to direct some healing his way.
One time when my dragonborn warlord fell in a pit and got separated from the party in the middle of a battle, and I was fighting a spider in the pit, I wasn't worried so much about my own survivability, as I was about my companions. With a good number of hit points, second best AC in the group, second best fortitude defense in the group, a high healing surge value, and access to two inspiring words and a few potions, I wasn't in any real danger. So I could argue in such a situation, a warlord would fair better than most.
But if we are talking about surviving raw number of hits over the course of a day, a Goliath, Warforged or Dwarf Battlerager or Earth Warden will do quite well. With Earthwarden, you can also multiclass into fighter and grab one of the nice regeneration powers, as well as a fighter paragon path like Dreadnought.