There are so many I can hardly make a choice:
Loads of cheap plastic gadgets that I bought "just in case they worked as advertised" - they rarely do. e.g. $2.99 ball compass that spun in circles for two minutes then fell off and disappeared.
Travelpro suitcase - Outrageously expensive (paid ten times as much as for its predecessor which lasted for six years of almost constant travel) and handle fell off the FIRST time I used it (held on with teeny self-tapping screws into plastic). Much vaunted customer service is also crap.
Joe Rocket Alter Ego pants - Zip-off mesh panel is a PITA and still doesn't provide much cooling; legs are too short (and I'm no lanky freak) coming half way up my calves with legs bent; waistband adjusters twist around and dig in; and the lower legs are made from low melting point material - duh!. Not entirely worthless in moderate weather but I could have bought a decent pair of cold-weather waterproof pants AND a pair of mesh pants for the same price.
I'll join in nominating the Bronco II (2.8L V-6) - In 3 years of ownership, I went through the original engine and two rebuilds; the A/C NEVER worked for more than a few days at a time; power windows and locks went out multiple times; brakes, driveshaft and various interior fixtures also gave trouble if I remember correctly. Had a seriously dud Ford Escort once too - Will never own another Ford product.
Avon Speedmaster tyres - I've never actually bought any but they were on several bikes at the time of acquisition and were all totally crap.
Have loved every motorcycle I ever owned although I'll agree that my 1979 XS250 US Custom was quite awful in a number of ways.