The quintessential 5e adventure is Lost Mines of Phandelver. I can't say there is anything exactly special about it. It's main virtue is that it is not so overwritten with bloat as the big book adventures WotC makes these days, sticking to something simple and loose that even a first time DM can riff on, expand on, and remix without too much stress or loose ends. It's second greatest virtue is that it efficiently delivers everything a first time player expects D&D to be. It is the thesis statement of 5e era D&D.
In any case, as the starter set adventure it is absolutely the "Keep on the Borderlands" of 5e, and the one that will be a nostalgic touchstone for a lot of the people who started in the 5e era, which is a lot of people. If there is a 5e adventure whose place in the pantheon of D&D adventures is assured, it's LMoP.