For me, has to be the 3DS. For three reasons...
1) Design Faults
From day one, I had trouble getting my 3DS to work on the Internet correctly. It took several minutes for Swapnote and the eShop to load, and half the time attempting to play online ended with failure. On one occasion, I tried three times to play Mario Kart 7 online and failed, tested my router by playing a game of Wipeout HD Fury (PS3) online without issues, then tried Mario Kart 7 three times again. The Swapnote slowness came to an end when Nintendo killed the application and replaced it with Miiverse, which is even slower.
The lack of a second analog stick could have been fixed by having a capacitive touchscreen like 99.9% of other devices now have, but that wasn't included. Instead the most fun you'll get out of Kid Icarus before your hand goes numb from RSI is probably the monotonous conversation scenes. In addition the motion controls didn't work properly with the 3D display, as when you tilted the device with 3D on, it messed up the effect (limitations of the 3D screen technology).
2) Awful Games:
No doubt a lot of people are going to disagree with me on this one, but the 3DS had possibly the worst set of first-party games on any console. From Mario Kart 7's broken gameplay and inability to play online to Starfox 64's poor replayability and bad motion controls, it seemed like Nintendo had completely bypassed quality assurance. Plus they charged £40 a time for games which had much less playability than even a 69p Android App.
3) Culture of Bullying and Intimidation
I tried speaking out about the issues I was having. The result was having all my 3DS owning friends turn on me, calling me nasty names, and claiming I was just a whiner who didn't know how to use the system (with being unable to connect to the internet) or a bad gamer (in response to MK7's broken Item Randomiser). They ignored the fact that every other console I had ever owned had none of the problems. How a console can turn people horrible defies belief.
Extra Note:
Two months after I ditched my 3DS, I was told by one of the aforementioned Nintendo fans that the reason the 3DS had issues connecting to WPA2 networks was because it wasn't originally designed to. If I had known that I would never have bought it in the first place.
After ditching my 3DS, I used the trade-in credit against a Nexus 7 (2013) tablet. I've not regretted my decision once.
Flash Sentry's #1 Fan (unofficially).
