What is the biggest obstacle you have overcome in your existence?

In my existence? I think I'll interpret that to mean my present incarnation, as that's the existence that's of immediate concern to me.

So. Given that understanding, the biggest obstacle I have overcome in my existence is – myself. Or rather, the supposed amalgamation of my darker side that calls himself the Valeyard. Not that I fully believe such a thing possible, especially when the claim comes from someone as disreputable as the Master. But when you've been around as long as I have you learn that even the impossible might be probable.

Irregardless of who he claimed to be, suffice it to say that the Boneyard Valeyard very nearly succeeded in destroying my life for the shear perverted joy of it. The fact that several corrupt members of the High Council of Time Lords promised him the remainder of my regenerations as reward for his services should he successfully prosecute me on a charge of genocide, didn't sway him from trying to vaporize them with a Particle Disseminator.

He very nearly succeeded in both. I can't say that I've ever had reason to thank the Master before, but I must admit that his rather surprising intervention and revelation to the court made all the difference. The old rogue gave me a new understanding of the situation that enabled me to second guess the Knackersyard Valeyard's intentions. I was able to thwart the plan but not before he managed to escape justice. I imagine he's out there somewhere, waiting for another opportunity to finish what he started. And I'll be right there, ready to challenge him.

To thine own self be true. But sometimes, you have to make exceptions.