I have never really owned a bike that I would consider to be bad. Even my £99.99 5 speed Apollo Road bike from Halfords (1989), which I consider to be my first bike (as opposed to a toy) was great because of what it meant to me at the time.
However, I have ridden some duff kit.
Pace RC36 EVO II's, long travel. Bluggh
Wrong fork for my Orange totally, may be a nice fork, just not on that bike.
Rock Shox Mag 20's, seals blew with alarming regualrity. Swapped them for a Control Tech stem and a set of Tange Big Forks. Much Better.
Tioga Factory XC tyres. What a pile of s**t. How to make any bike feel awful. Came as standard on my Mrs' Kona Nu Nu, changed them for Panaracer trail blasters and saved 1 1/2 lbs off the bikes weight, and made it nice to ride.
When they first came out I had a GT RTS3 for a weekend as I was intrigued, nearly bought one, glad I got the demo week end as it saved me a lot of money.
Those the the only things that I have really disliked, everything else I could live with.