With "Essential" I'm going with what I know of that has reached a huge scope of ordinary people who listen to music regardless if they are musicians or not. The following list is my perception from what I've experienced from people outside of the Stick community:
1. Discipline.
2. Gordian Knot
3. Falling Into Infinity - by Dream Theater
4, Before These Crowded Streets - Dave Matthews Band
5. Tie between Larry Tuttle's "String Planet" and Jeff Pearce's "Rainshadow Sky"
1-3 Are mentioned to me very frequently whenever I'm introduced to musicians by other musicians that tell them I play Stick - or - among serious prog rock fans who are not musicians. The top three become a focal point of discussion.
2 Most hardcore metal heads I've spoken with bring up Sean Malone when I tell them I play Stick. I've had former coworkers who played in death metal bands share bootleg recordings of Cynic with Malone playing both bass and Stick, and thus Gordian Knot is mentioned. Perhaps Focus by Cynic would be a tie for the number 2 spot with Gordian Knot, but I've never heard the album mentioned, so I did not list it.
3&4 unfortunately only have one song on those albums with Stick, but "New Millennium" is a song I find most younger bass players, or serious Dream Theater fans identify the Stick with. Dreaming Tree has a bit of resurgence after Matthews created a line of "Dreaming Tree" wines.
5. I bring up because currently Larry and Jeff are the only two Solo Stick artists I know of that get regular airplay on SPA Radio. I hear them weekly whenever I listen to that station. I keep hoping some day SPA will have a weekly "Stick Hour" featuring other Stick artists
I'll leave it at that, my list would change drastically depending on one's interpretation of essential, which is already covered by a lot of other answers to this post