Wii U by far for me. It had:
- The best 2D platformer I've ever played (DK Tropical Freeze)
- The best action game I've ever played (The Wonderful 101)
- Two of the best open world games I've ever played (Zelda Breath of the Wild and Xenoblade X)
- The best couch co-op game I've ever played (StarFox Zero)
- One of the best Hack N'Slash games I've ever played (Bayonetta 2)
- One of the best and original competitive online games I've played (Splatoon)
- The turn based JRPG with the best combat system I've ever experienced (Tokyo Mirage Sessions #FE)
- One of the most original and immersive survival horror games ever (ZombiU)
- The biggest Mario Kart game (before 8 Deluxe)
The Xbox didn't have a single "best something something ever" for me. But it's not just that. No, no, no... the Wii U had much more going on for i. It was also backwards compatible with the Wii so I could play all of its games; it had a pretty big Virtual Console that allowed me to experience tons of classic games; Off-TV was incredibly handy not only for gaming, but for other activities like watching Youtube, Netflix or browsing the internet (I have probably over 1000 hours spent on that alone); it had lots of great indies like Shovel Knight, The Swapper, Pushmo World, Affordable Space Adventures... and while they were not very numerous, it had some great multiplats like Tekken Tag 2, Black Ops 2 and Darksiders I and II.
The Xbox was not a bad console at all. It had some great exclusives in Halo 1, Halo 2, Fable, Morrowind, The Chronicles of Riddick, tons of multiplatformers and graphics well beyond what the PS2 and GC could display. But to me all of that falls VERY short on what the Wii U gave me.
