1. "Beyond Logic" - episode 10
Of course the season finale would be the best episode. This is the culmination of the past 9 episodes, this is where we see Keiko reunite with her friend and newly-discovered family. We get much-needed closure of where she was, a sweet introduction to how things work in Hollow Earth, and we also see how APEX comes to fruition: with a nudge from Monarch and May's old boss from Applied Experimental Technologies, Brenda. There we learn that APEX wasn't born - that's AET being rebranded and repurposed.
This episode has a lot of emotional moments as Keiko deals with multiple bad news at the same time: she's been stuck in Hollow Earth for 57 years, BIll Randa has died, Hiroshi grew up without a mother, Lee is super old now, and she's a grandmother. Someone please hand her a glass of hot sake - that's so much to take in and process at the same time. Especially since Lee does not make it out of Hollow Earth with the pod, that's another loss she has to deal with after just seeing him for the first time in... well, 57 days for yet, but nearly 6 decades for him.
The emotionally charged exchanges between Hiroshi and everyone he's wronged, Tim putting his foot down and leaving Monarch (only to apparently join back up two years later) - those are pivotal moments not just for the show but the entire MonsterVerse lore as we know it. Things are getting carved out, and we get to watch the whole process.
Hey, AppleTV, can we have season 2 yet?