Nintendo: The GameCube. Many of Nintendo's key series went off in very experimental directions, some of which I didn't care for. Super Mario Sunshine in particular remains to this day the only mainline Super Mario game I never cared for. The GC wasn't a bad system, and was still pretty good with some solid titles, but it has always been my least favorite Nintendo console.
Sony: The original PlayStation. Honestly, I only had that system because of Final Fantasy VII. I had no interest in it beyond that. I did play a fair amount of games on it besides FF, but overall it was the poster child for the very painful transition from 2D to 3D. Polygon-based models looked dreadful on the system, even worse than the N64's. The system's controller didn't even have analog sticks originally, and this was almost certainly a major contributing factor why so many games on the system had dreadful controls. A lot of people look back wistfully on the controls of PS1 classics like Resident Evil or Tomb Raider, but I thought they sucked. Too many games were initially designed with D-pad controls in mind, but 3D games took little time at all to show why the D-pad was immediately obsolete for such titles. Camera's were frequently terrible, not just on the PS1 but for many early 3D games in general regardless of the system they were on. Gen 5 was a clunky generation overall, with very few games made where the developer had a clear sense on how to make good use of these new 3D graphics and the gameplay changes they frequently necessitated. Overall, I think Nintendo and Rare by far had the best ideas for successfully transitioning (and Nintendo had the sense to give the N64 controller an analog stick from the get-go), and as a result I felt the N64's library, while not numerically superior to the PS1's, was in terms of quality better. As a result, the PS1 was not a terribly impressive console in and of itself to me, and the PS2 was a step up.
Microsoft: The original Xbox. Honestly, there wasn't much on this system that interested me beyond Halo. If the PS1 was "the system that had Final Fantasy on it," then Xbox was "the system that had Halo on it." The rest of its exclusives weren't anything that could have sold the system to me. It wasn't a bad system, but it simply had no appeal to me beyond Halo. Had MS never bought out Bungie, I would have never bothered with Xbox (and given Halo CE's insane attach rate, a lot of other people likely would have ignored it, too).