Chapter 1 as that is where the fun begins
I nominate Chapter 77
For 2 reasons.
1) Is the emotionally rich stuff between Miata and Clarice, while they fight a hopeless battle against Agatha
2) The sheer awesomeness of the Warriors from the North's appearance at the end.
Chapter 57 The Invasion of Pieta 1 (Rigardo's Assault)
There are many good chapters in Claymore but this was so unexpected, so shocking that I think it deserves a nomination.
Chapter 113 Having killed off several characters and introduced more warriors and more awakeneds by the dozen, at one point I was wondering where all this is going and whether Yagi lost motivation. And then comes this chapter, a standalone that isn't a real standalone (where am I quoting from?) which re-establishes Miria's awesomeness on a whole new level, and when she appears towards the end you almost hear film music...