For me it is the hundreds of Virtual Console games I bought on it.  All those Metroids, Zeldas, Marios, CastleVanias, Meg-Mans, Kirbys, ect, ect, ect

That is why the WiiU wins for me. A 4 TB double hard drive case with a crazy amount of Virtual Console and 3rd party digital games.

Personally I was never able to get into the 3DS; while it did have some good games like RE Revelations, Mario 3D Land, and Monster Hunter 4, they always felt hampered by its uncomfortable controls and tiny low quality screen.

Wii U for all its faults had more great games like BOTW, Bayonetta 2, Mario Kart 8, Splatoon, Mario 3D World, and Tropical Freeze, and was more pleasant to play in terms of both tactile and visual comfort.

Wii U for the porn alone

It was one of Nintendo’s best/worst consoles.

Of course the 3DS has the better library, but if a game exists on both consoles, I'd often opt for the Wii U version. Portability is nice, but I rarely play games away from the comfort of home, so I might as well be playing with a much more comfortable controller and a far bigger screen.

3DS. The Wii U had great games and Nintendo definitely tried really hard to make the Wii U a success, but the fact is that you can have almost the entire Wii U experience by playing the sequel games that all came out on the Switch. There is almost no experiences left, except for maybe Xenoblade Chronicles X, that are unique to the Wii U now. On the other hand, the 3DS had an amazing library of games (especially third party RPGs) that will probably never come out on other consoles or eShops thanks to the second screen constraint. Furthermore, the online support for the Wii U is disappearing fast which makes many of its best games barely playable. The 3DS can one day be an awesome retro gaming handheld, the Wii U will just fade into obscurity for the reasons mentioned above.

WiiU was a great stopgap console. If technology had been a bit further along (or affordable for the Nintendo price) than it would have been a guaranteed success. The initial speculation regarding Project Café oversold the concept, but they finally delivered on that concept with the Switch.

It was a great time for indies, Nintendo dipping their toes in HD development, and created so many iconic Nintendo titles that are currently getting a second life.

I think the marketing and the time and the faddish nature of tablets as well as a Sony that came out swinging with the PS4 limited the success of the WiiU, but I’ll always have fond memories of the system.