What is the second hardest block in minecraft?

Paper

See chart for All hardnesses.

http://minecraft.gamepedia.com/...y_hardness

I'd say either end stone or obsidian.

The second most durable block in Minecraft that you can obtain in survival, assuming that's what you mean, is End Stone. It has a blast resistance of 45. The most resistant is Obsidian which I think has 1,000. All other blocks have 30 or less, including but not limited to Stone Brick and Cobblestone.