Picard: 15 ESSENTIAL Star Trek Episodes To Watch First (And Why)
14. “Datalore” (TNG - Season 1, Episode 13)
It may not be the case that Data is even in Picard all that much - we've seen in the trailers that he's involved in at least two sequences, but one has been described (by Brent Spiner himself) as a flashback and the other appears to be a holodeck simulation of a chess game - but he's an important driving force behind the plot all the same.
Because of his sacrifice, Data is part of the show's emotional heart and a major part of Picard's own motivations to get back into the game. This is not one of the finest Data episodes, but it does offer some insight into Dr Noonien Soong's work with androids and their imperfections - as well as establishing that he was discredited - and it is such an important part of Data's past that it feels impossible to discount.
It also goes some way to establishing why there's another Data in a drawer in Picard for anyone who might not have paid attention to TNG and Nemesis.