Starting off with a shoutout to the father of the skyscraper, coiner of "form follows function", and mentor to Frank Lloyd Wright: Louis Sullivan.
I think his best work is the Sullivan Center (also the Carson, Pirie, Scott and Company Building). I think this building is a stellar example of early movement into the modern open structure of large structures we continue to build today.
Its proportions and lines are classical for sure, yet not a single one is unnecessary and out of place. The use of steel (still uncommon in 1899) allows an incredible amount of window area, a pioneer in an age of buildings made from stone. And the detail on the facade is just amazing, all done in cast-iron pushed to its limits.
Next up Toyo Ito and the Sendai Mediatheque. This building is like a small forest, the structure is all in those bendy trunks while all the floor space is the tree canopy.
Each floor's program is incredibly flexible too, intentionally so. It's like a combination library, video store, record store and event space. But people seem to like it so there you go.
I've never been to this one but the High Line in New York is neat. Old converted elevated rail line turned into a very long very thin park.