In my other post in this thread, I said Xbox 360. While a loser, it was still what I'd consider a huge success.
My favorite inarguable loser is definitely the Dreamcast. I actually hated Sega and I was happy to see the Sega Saturn so soundly stomped by superior systems. Fast forward to early 2001 and no matter what I did, I couldn't get a PS2. Frustrated, I finally gave up and bought a Dreamcast. What a fool I'd been!
This thing blew away my every possible expectation. I was buying great game after great game. Graphics were light years ahead of what I'd experienced on N64 and PS1. The machine was full of exciting innovations, too. Maybe a week (that's how I remember it) or maybe a month after buying my Dreamcast and bragging about games like Shenmue to my PS2 owning friend, I watched sadly as Sega announced they were leaving the business but still supporting the Dreamcast for one year. Then the price dropped $100--a lot of money for me at that time.
In March 2001, I finally got my own PS2 with copies of The Bouncer and Zone of the Enders (with NBA Street, Red Faction, and Onimusha to follow). After a couple of weeks, I was right back to my Dreamcast. Say what you will about the PS2 but up until maybe November 2001, no PS2 owner alive was having as much fun with their console as I was with my Dreamcast.