My worst CPU was DEFINITELY a K5-PR133!
I used it for two weeks or so only to throw it away then in favor of a Pentium 133, which turned out to be my second-best CPU ever (used it for 2 1/2 years...it ran great).
While AMD's K5 was a piece of crap (only marginally faster than the 486DX4-100 I used before), I consider a good old am386DX-40 to be the best CPU I ever had. It also survived several years and when the 486-line of CPU's started to dominate, it hold its ground very well against the "Celeron of the past", the 486SX.