Among skyscrapers, one fave (among many) is
225 South Sixth by Pei, Cobb, Freed:
The rest are houses. Houses are where real architectural ideas are tested out, since they usually do not have to adhere to commercial constraints. It's hard to limit the list to 5 however! I could also include LeCorbusier's Villa Savoye, Adolf Loo's Villa Muller, Pierre Charreau's Maison de verre, Reitveld's Schroder House, etc.
Bruce Goff's Shin'enKan: (Sadly destroyed in a still-unsolved arson fire in 1996)
Bart Prince's Joe Price House:
Mies van der Rohe's Farnsworth House:
And, of course, Wright's Fallingwater: