This is SO hard, but my heart has to go with S1 E6: The Three Bad Wolves. While I love the sophistication of the later episodes, I have a fondness for the early days of Nick learning this whole new world. I also love a Monroe-centric episode, and his early time with Hap is hilarious. But Angelina (Jaime Ray Newman) is Grimm at its best, giving women its smartest, most powerful roles.
Runners up: I mean, omg, all of them (except for The Thing with Feathers, Kiss of the Muse and Bad Moon Rising)? But if I have to choose, maybe:
S2 E2: The Kiss – Kelly kills Katherine (another spectacular older female character) in spectacular fashion
S2 E22: Goodnight, Sweet Grimm – Oh, love me some Papa Ghede!
S1 E22: Woman in Black – the introduction of Kelly Burkhardt!
S1 E3: Beeware – the culture of the Mellifers was fascinating
S1 E21: Big Feet – for Monroe’s “Larry?”
S1 E5: Danse Macabre – I love Roddy!
S3 E19: Nobody Knows the Trubel I’ve Seen – introduction of Trubel!