Celebrim
Legend
So this is much harder than I thought. For now, I am landing on these-
B4- the lost city. This is the perfect introductory adventure. Not as iconic as Keep, but better in every way.
I3- a module that is so good and so different that I can still remember buying it and reading it while the Thompson Twins played on a radio in the background.
S3- like peanut butter and chocolate, sci-fi and fantasy go together perfectly in this module.
WG4- Thar. Iz. Dun.
X2- perhaps my all-time favorite adventure, ever. It has everything, including a weird Lovecraftian basement interlude.
Good choices.
I seem to be the other "I3" voter in the poll. I also voted as you did for X2, though I don't like it as much as you do, I do see it as an early predecessor and template for the sort of modules I do like. I nearly voted for WG4 but didn't because it would only be for the atmospherics in the second section, and I admire B4.
I think the only one here I disagree with you over is S3 - I think S1 is the superior adventure in the S series, and that was one of my 5 votes.
I3 is the sharp dividing line between the very simplistic dungeon crawl scenarios of the "A" series, "G" series, B2 and so forth (that I don't like), and the more complex story based dungeons seen in UK1, I6, etc. (that I do like).
I voted U1 as a prototype of that "story based dungeon" and still an excellent starter scenario. It's let down in my esteem though by the lack of really good guidance on how to follow up on it.
I also voted L1 as an excellent starter scenario, and because I have a good deal of admiration for the follow up: L2: "The Assassin's Knot".