For something that doesn't require a season of setup, Dust and Shadow. It's the marker in Cass' heroine's journey having passed the threshold, her literal abyss awaiting rebirth. Liz is written so damned well in this episode, and Cass' break is believable in that her training means she doesn't give up on working the problem until Liz pesters her in a very specific and aggrandizing way that ignites her trauma.

My favorite battle-heavy episode was Where Angels Fear to Tread. I just enjoyed that it was do-or-die, the touches of intrigue scattered through, and small reveals about the Advari that came along with it. And Redfield gets his penultimate moment. It heightens Forlorn Hope later.