The GC, and it's not close.
The one time they had possibly some relevant tech advantages over Sony's console, and they squandered it all with the laziest, most rushed, most unpolished games for their main IPs. Launching with Luigi's Mansion, a game that's over in a couple of sittings and that spends most of it in repetition. Mario Sunshine is the worst 3D Mario by a landslide. The Wind Waker needed a lot more time in the oven, and even then, its basic concept of sailing through an ocean divided in perfect squares containing 99% of nothing was terrible.
GBA connectivity? Way overrated, and at a price that makes the Joycons sound cheap.
Their most gimped, less versatile controller ever. Imagine having to buy a third-party controller to get a decently-sized Dpad of all things.
Overrated third party support. The games that really counted never released on the GC, and most of the best third parties also released on PS2 anyway.
Imagine thinking that the GC's first- party library is better than the Wii's when the Wii had Galaxy 1 and 2, Xenoblade, the best Wario platformer, the best version of Twilight Princess, and many more, along with ports of GC first-party games that significantly improve on the originals (Metroid Prime with the Wiimote is leagues better than the original, it's a fact).
I hate how Nintendo had so much potential with the GC, and just didn't know what to do with it.
The one time they were seriously competitive in tech, and they squandered the chance with terrible executive decisions and an obvious lack of direction.
