The Old Mixer wrote:
Rhaenys seems to have hit all the highlights. OTTOMH, I'd add "Silicon Avatar" to Season 5, as it ties into Data's origins and one particular aspect of his nature that was otherwise unexplored in the series.
Yes, that is a good suggestion, too.
The most important episodes are in my previous post, but if you want more episodes, you could watch Peak Performance, from second season and Deja Q in third. They are not entirely Data episodes, but they show some aspects of his character. In season four you could watch Legacy. And Masks from 7th season, it's not a very good episode and it's not really about Data, but it shows a very interesting experience for him.
And if you want to see Data in command I suggest watching Redemption, II from season 5, and Gambit from 7th season.