I did an article about this last night, but I'm interested in reading your thoughts.
I had chose:
Rayman Legends - one player interacting with the levels to assist another player or players
Wii Party U - An abundance of really cool games and modes that utilize the GamePad in some inventive ways. My favorite is probably Name That Face, a three + person mode where the GamePad player gets a description and they try to best create an expression with the controller's camera, taking a photo for the players. The players then select from four answers what the GamePad player was trying to express with his or her face/hands. Then there's the tabletop games, played solely on the GamePad.
Deus Ex: Human Revolution - Director's Cut - puts many gameplay concepts to work on the Wii U GamePad without it feeling shoehorned in-- things like hacking, inventory management, 3D maps, etc.
Nintendo Land - pretty obvious pick; showed multiple uses for the GamePad
ZombiU - Cycling through your inventory on the GamePad screen while the game is still active is quite intense. A zombie could come creeping and infect you at any time. Then there's passwords that can be inputted into the touch screen and helpful maps.
Since I was doing a top five list (always fun to write-- no sarcasm intended), I didn't get to include LEGO City Undercover's scanning of areas to look for criminals, vehicles, and hidden goodies.
I hope I didn't just list the only games that make worthwhile use of the GamePad. Even if I did, which are your absolute favorites?