Freddie showing up at the train station and proposing to Ada. It's going to gobadly, but it's sweet.
In episode 3, the look on Freddie's face when he sees Ada in her wedding dress. He tells her she looks like an angel.
Finn falling asleep on Polly's lap.
Polly attempting to get Freddie and Ada out of England multiple times, trying her damnedest to avoid the bloodshed that will occur if Tommy has to deal with the situation.
Tommy buying the Garrison and giving it to Arthur to run. It'll help with Arthur's PTSD and depression over being basically superfluous to the Peaky Blinders (now that Tommy's in charge), and Arthur's apparently always wanted to own a bar.
Tommy's attack of conscience over whoring Grace out to Billy Kimber. The precise way he gets her out of sleeping with Kimber may be embarrassing for her (he lies and tells Kimber she's got the clap and that's why Kimber can't have sex with her), but the fact remains that he realized Grace shouldn't just be a pawn to him.
How panicked Tommy gets when he realizes where the Lees have wired the grenade, and his frantic attempt to get Finn out of the car. Even though the wire gets tripped and the grenade goes off, Tommy scoops Finn up protectively.
John's wedding, after the arrangement part and before Ada picks a fight with Tommy. Peace is mostly achieved between the Lees and the Shelbys, and everyone's happy and drinking and dancing.
Ada giving birth to a healthy baby boy, and Freddie holding his son for the first time.
Tommy "not hearing the shovels" (a part of his PTSD) while laying in bed with Grace after sex. Sure, they seem kind of doomed, but it's still a sweet moment.
Polly calming baby Karl for Ada, sharing some mothering tips. Takes a swerve into Tearjerker, though, when Polly tells Ada just what happened to her own kids.
Ada showing off baby Karl (for Marx, of course) to her family and the rest of the Peaky Blinders, and Arthur racing over to hold the baby. He even puts his cap on the baby's head.
Freddie deciding to join Tommy and the Peaky Blinders in the battle against Kimber, despite Tommy getting him thrown in jail the night his son was born and trying to kill him.
Grace telling Tommy she loves him and asking him to run away to New York with her.
The three boys and Polly getting drunk in the Garrison together after the battle with Kimber's men. Polly says it was a good day and that there will be others, and Tommy breaks out the champagne he'd had Grace save, toasting "to the others".