I have to go with two episodes. There's two things on Berserk that made it very special to me: First is the mix os styles, and on episode 118: Beast of Darkness we can see that, first the panels with the forest burned to ashes after the battles with Rosine, there are many child corpses burned on the ground(one of the most dark moment of the series), and SK walks in, appear on a beautiful panel on top o his horses and start a monologue about Guts and.. EAT A BEHELIT(one of the greatest misteries that we see explained), then Guts, walking covered in blood hearing spirits talking about "his fault" about the dead children around, these spirits gradually transforming into the Beast of Darkness then gaining Guts face shape, and then Femto head/helmet, tormenting Guts until he lost the control and fall to the ground and star to think about his objetivo that time: revenge(we can see he's suffering) then Puck arrives and the climate drastically changes, he really save Guts life here to the point that Guts has to accept it, and even call Puck by his name for the first time(the type of comedy that perfectly balance the story), and on the last panel we see The Holy Iron Chain Knights without even imagining who the fuck they are.
These drastical changes of tone is the second main reason(obviously, there are many others).
The second chapter that I have to choose is 130: Fragile Flame where Guts decide that he will never lose her again, that's one of the more heart breaking episode for me, I can sense his suffer because Casca disappeared, his face expressions thinking about this and remembering the eclipse, the first time a read it I remember thinking if he can accomplish these promise or if more and more disaster was what is expected by his journey.
Miura's skill to deliver emotion to the readers are awesome, and on my opinion it's unique and perfectly done.
Both episodes transmit the evolution of Guts way of dealing with his trauma, and both are turning points on the story, obvious the greatest turn point was the events on the eclipse, and I agree with Walter about episode 78, but for me that started reading Berserk in 2008 these events was a bit spoiled(mainly that Griffiths turns a villain), and so i wasn't very surprised by Griffiths confirmation about the sacrifice (sadly). So for me, these two episodes was the ones who i remember the most of my first read, and every time I re-read then the same feeling returns even if I know what comes after. Maybe after read the later episodes made these two even more important and emotional to me.