Obito's. At least Nagato had the comfort (if you can call it that) of knowing that Yahiko was happy to die for his comrades. Obito likely believes that Rin's death was unjust, unnecessary and avoidable. You could say the boy was obsessed with Rin (and that this was one-sided), but that wouldn't make it any less painful.
No offence but what makes Yahiko's death just, necessary and unavoidable? Or his parent's death?
I agree that Rin's death was painful but still not a powerful reason as Nagato's.
If Yahiko was not killed, Konan's life would be taken. There was no other way out of that situation unless Nagato was willing to sacrifice Konan's life. For all we know, Obito thinks Rin's death could've been easily avoided had, for instance, Minato been there to save her.
We still don't know the circumstances of the whole Rin death, so until that is unveiled, still Nagato's painful.
And even if it was Konan who would've been the one to die, still doesn't make her death just and necessary. Plus, don't want to be bitchy or anything, but Obito got mad over a girl, but could care less that he kills his own clan, from what we may suppose, as far as we know, his parents could've been there.
Not to mention the children.
Nagado did everything because of the pain he suffered after he lost his parents and his best friend.
Now, think as this... Who would you suffer more for? Your parents or your loved one?