Mario Kart 8 and all the DLC.
How cheap are you getting it? It might be worth your while (after crunching the numbers) to get the Kart bundle since DLC is included. They should still be pretty easy to get.
Otherwise, I enjoyed Star Fox Zero, but try that one out first if you can. If I wasn't a Star Fox fan I can't say if I'd recommend it.
LEGO City Undercover is good fun.
Star Fox Guard is very good.
Bayonetta 2 (see if you can get the copy that comes with Bayonetta 1 included) is also great.
Captain Toad Treasure Tracker is a ton of fun and probably one of the most polished games I've ever seen.
Rayman Legends is good.
New Super Mario Bros. is great if you like platforming.
Kirby and the Rainbow Curse is fun, easy, and gorgeous to behold.
Wonderful 101 is hard and fun and hard.
Donkey Kong Tropical Freeze is a must-own game for this system.
Super Mario Maker is also worth getting, even if you don't love platformers.
Those are all the games I own that you didn't already mention. You may also want to look into some choice Wii games like A Boy and His Blob, Okami, Punch-Out!!, and Excitebots Trick Racing. Also also, if you already own a Wii, make sure to hang onto it so you can do a System Transfer. You'll be able to re-download anything you purchased on that Wii onto your Wii U.