Some of these weren't great, but probably don't deserve to be on this list.
RDRAM failed, not because of RDRAM, but because of the Pentium III. Which brings us to the Willamette too. The Williamette reached 2 GHz on the same process technology that the Pentium III/Coppermine reached 1.1 GHz, and outperformed it EASILY at virtually everything at that clock speed, and even when introduced at 1.5 GHz (compared to 1 GHz Pentium III) beat it in virtually all benchmarks. Why? A good part of it was the performance of RDRAM, which finally was attached to a processor that could use the bandwidth. The bad was the x87 was greatly weakened so developers would use SSE 2, which was very powerful and considerably better. I guess that's a plus and a minus.
Ironically, RDRAM died just when it was finally better than the competition. The price had finally come down to the price of other memory, and the performance was better for the Pentium 4 than DDR. But, when they released it for the Pentium III, which couldn't use the bandwidth (although even then, the i840 had very good performance by using interleaving to reduce latency), the performance was bad because of the processor. The cost was excessive too, although it was wrong attributed to royalty payments.
While the Willamette wasn't a great processor, the really bad one in the Pentium 4 line was Prescott. I think most people would pick that one as the worst.
Others might be all the Super 7 chipsets. The 386 kind of sucked (added a lot, but the performance wasn't that great). The K5 was a big headache, being very late, lacking MMX (which was a selling point by the time it came out, due to be late) and stuck at 116.7 MHz. AMD 486/DX2 80 MHz had that nasty tendency to have the internal cache go bad. i820 really sucked, being late, using the wrong memory (RDRAM) for the processor, and then having bugs with the MTH. Worse than that, for only a little more, the i840 had much better performance, and more features. Cyrix should have been known as Cryix, because you would if you bought one. The headaches those things caused ...