I have three:
-Gamecube: How could a console that had the best version of Soul Caliber II, Metroid Prime, Wind Waker, Resident Evil 4, Mario Sunshine, Pikmin, Smash Bros. Melee, Super Monkey Ball, Resident Evil Remake, Viewtiful Joe, Baten Katos, Paper Mario: TYD, and so many others sell so poorly? It has never made sense to me!
-Atari Lynx: Considering that this came out the same year as the monochrome Game Boy, this handheld is ridiculously impressive! It had Mode-7 style scaling before SNES was even launched, and even though most of the exclusives were lackluster (Dirty Larry notwithstanding), it had some really great/faithful arcade ports with Double Dragon, Ms. Pac Man, APB, Toki, Pit Fighter, etc. One thing that killed it, though, was it's appetite for batteries!
-Dreamcast: I could sit and talk for hours about this console, and the many theories as to why it failed. It had all of the ingredients for success: an attractive design, third party support, an incredible launch in the states (Soul Caliber, Sonic Adventure, Powerstone, NFL2K, etc.), and Sega putting some serious marketing muscle behind it. It was never going to dethrone Sony, but Sega could have come in third, or even second place!