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The Wii U by a mile. People rate the Wii U too highly right now because of recency bias. But there never was a slower, more boring and barren system than the Wii U: The console only had a handful of good games, most titles were very niche but have been hyped up due to the lack of alternatives on the system. Game&Wario was worse than previous Wario Ware games, Mario 3D World was worse than Galaxy, New Super Mario Bros. U felt stale after the onslaught of NSMB titles, Nintendoland doesn't hold a candle to Wii Sports and Wii Sports Resort, Wii Fit U was almost the same as the Wii game, Starfox Zero is the worst Starfox game in existence, Tokyo Mirage Sessions certainly didn't captivate as many players as a proper Fire Emblem would have and Wonderful 101 just has no market appal outside of the hardcore gaming crowd.
Add to that the fact that the hardware was atrocious and you have Nintendo's worst home console by far. And I don't mean the CPU power, I'm talking about the slow system, menus and downloads. And the gamepad, of course... the most unnecessary, needlessly bulky and overstuffed controller created in the history of mankind, which not even Nintendo themselves found a proper use for in most titles.
The Gamecube comes in second, but there's a clear distinction between the first two years of the system and the latter half. At first, the Gamecube looked promising. It had many good first party titles like Smash Bros, Metroid Prime, Pikmin, Luigi's Mansion, Eternal Darkness and so on. And decent third party games like Rogue Squadron II, Sonic Adventure 2, Resident Evil Remake and Zero and Need for Speed Underground. But after the first two years, support for the system just dropped and dropped and 2005-2006 was a barren wasteland of releases.
Overall, the Gamecube definitely has the edge over the Wii U. The Gamecube's hardware was better designed, the controller was superior to the Gamepad, the console had more first and third party games and while a lot of titles were a step back from their N64 counterparts in many ways, they were still of solid quality, which really can't be said for Devil's Third, Mario Tennis and that Animal Crossing spin-off. The Wii U didn't even have its own Zelda game prior to the Switches launch and two of its most highly rated games, Twilight Princess HD and Wind Waker HD, are Gamecube remakes.
All in all, he Gamecube deserved its low 21m sales. But the Wii U deserved its even lower 13m sales just as much.
