OK, my essentials partly match yours, but I have some other favourites as well.
1. M.U.L.E.
Best played on a real Atari, to have the fantastic muliplayer experience, where you can learn, that you have to compete and cooperate to win.
2. Koronis Rift
Fantastic game, big worlds generated in real time wait to be explored.
3. Dropzone
Best Defender clone ever. Period.
4. Star Raiders
I love this game, the beginning of all Space Simulations and still worth a try, where else you can fly with damaged engines, broken lasers and a flickering shield through an asteroid field?
5. Karateka
First game that played like a movie and the end will surprise you, I promise.
6. Archon II
Great adaption of Chess in an action game, the second part is even better, then the first one.
7. Ball Blazer
With one or with two players, the best future sports game you can imagine with an 8 direction digital stick with 1 button.
8. Mercenary
First open world game. Must be played to be believed.
9. Jumpman
The main character is only 4 or 5 pixels high, but after a while, you will love him.
10. Boulder Dash.
Invented a whole genre, fantastic action puzzler, not to be missed by any chance.
There are many many more great XL/XE games, the more I think, the more come to my mind, but I think this is my essentials list.
