"Assignment: Earth" (TOS) --> Gary Seven, the Supervisors, and The Watchers. "Where No One Has Gone Before" (TNG) --> Picard's mother. Wesley being introduced to The Traveller. "Family" (TNG) --> Picard's brother, who's just like their father. "Tapestry" (TNG) --> Even though "All Good Things" is a more obvious choice, this is another time where Q makes Picard confront his past. And they share a bed at one point. How is this NOT a prequel to Picard Season 2? Not Including (for myself anyway): I've seen Star Trek IV too many times. I can quote the entire script without looking it up. "Past Tense" (DS9) has them going back to 2024 but that's where the similarities with PIC Season 2 end.
"Child's Play," "Imperfection" and "Shatteted" from "Voyager" all add extra weight to Icheb's murder, and make Seven's revenge even more satisfying.
I'm adding "Tomorrow Is Yesterday" (TOS) to my list. It shows the sling-shot effect in all its glory for the first time, one of Earth's early space missions, alludes to a mission Captain Christopher's son will lead close to the time PIC Season 2 is set, and I just feel like watching it again.
That's more a thing for heading into Season 1. Before Picard premiered, I re-watched every episode and movie of TNG, along with every prominent Seven of Nine episode of VOY. And I was glad I did. I got more out of PIC Season 1 than I would've if I'd just stuck with my faded memory. While I'd previously re-watched all of VOY in 2008 (or watched for the first time when it came to the last two seasons), I hadn't seen the vast majority of TNG since I was high school, in the '90s. Even in the case of VOY, it had been a really long time.
Tapestry is one of my all time favourites. Maybe DeLancie's best performance as Q as well. "You're dead" It's one of my most seen TNG episodes.