The original Xbox.
Back in 2003, I had to make a difficult decision. For the first time since the Genesis (which I didn't even own anymore), I was going to buy a non-Nintendo console. I made the tough call to pick up an Xbox for a handful of reasons, including Halo 2 parties, the upcoming KotOR, and the fact that a good friend of mine had a PS2 and I thought it'd be better to get something different.
Ended up not really liking either of them. Halo 2 became the game that officially caused me to take a general disliking to the FPS genre -- nothing specifically against the game at the time, it just became a type of gameplay I no longer enjoyed, which continues to this day. As for KotOR, for whatever reason, just couldn't get into it. I didn't enjoy the combat, and the story didn't manage to engage me enough. Got bored about 10-15 hours in, and never went back.
Ultimately, I only bought about 6 or 7 Xbox games, and it turned into my Jade Empire, Vice City and Ninja Gaiden box, which are the only games on the system I actually played through to the end. Ninja Gaiden remains one of my favourite games ever, so that is the one saving grace of the system, in my eyes. Overall, I regret buying it quite a bit. PS2 was what I needed, and I eventually wound up getting one in 2007 (AFTER buying a Wii!) and then a PS3 in 2008.
Still don't have a 360...
Edit: You might be saying, "But you sold your Genesis! Surely you regretted that?" Well, no, the problem was I never bought any games for it. I just rented a ton. Eventually, when I was moving on from the 16-bit era, all I had were a lame light gun thing and Sonic 2. My dad just decided to trade it in.