Berserk: The eclipse. I'm not going to say more than that. If you've watched/read Berserk then you'll know exactly what I mean. If you haven't, I'm not going to tell you as it's the defining point of this series and to spoil it would be to ruin the entire experience.
One piece: The first spoiler Redeyephantom mentioned, plus the bit where Nami is trying to remove the tattoo on her arm by stabbing it repeatedly, then breaks down crying and whimpers "Help me" to Luffy. Oh, and Luffy's massive war cry afterwards was cool too.