Been lucky to own a lot of bikes and ride even more.
Some of the offenders that I wanted to like: (to be fair most were new or close to it or in exceptional shape, and the tire pressures etc., were correct)
Cagiva 650 Alaruzza SS. I owned this for awhile, sort of a Mini VFR - wooden brakes, nice power band, but real slow to rev, a bit heavy and underpowered - surprise, very reliable and well put together. Bonus: Best sounds with the Bubb pipes.
Guzzi V50 Monza, loved the bike for its size, handling and light weight, super stable, but no torque, no real power, terrible way too wide gearing, which of course could not be changed.
'78 Honda CB750F. This was one of the Japanese bikes that were now supposed to handle. The flops over in corners kind of handling, required a lot of something to get it back up, what a chore. Felt piggish to me.
'99 or so Laverda 750 - their attempted comeback. Sweet handling, brakes, looks, but it was withering (in a bad way) when you realized the engine is just a larger 1970 Honda 350 that behaves, and sounds, just like one.
Kawasaki: EX500, mush all around, everything felt like mush. It was new too.
2008 Kawasaki EX250 - nicer than a EX500, but a chore to rev and ride, felt heavy. I like the 1994 - 2007 versions better.
1998 Honda VTR1000 - lovely engine. Twist and go, then get gas at 85 - 100 miles. Brakes pretty good, but another flopper in the corners, same heaviness to rightitself.
2008 Kawasaki KLR. I bought it, total crap. Brakes, power, vibes, uncomfortable, crap rusting as you watched. Good range though - but that made it worse, having to put up with misery for so long at a time.
1997 or so Honda CB750 Nighthawk - boring, everything felt muted and vague.