The only "quality" KOs I have are the Dino cassettes. I'm glad I got them (because, obviously, unobtainable otherwise), but they are not really great copies. Some of the details are shallow, some of the joints are super loose, and the stickers do not really fit the figure well (wish they took the suggestion to go with tampos).
I have a lot of Classic Seeker KOs, and the Punch/Counterpunch KO. Seekers due to army building and P/CP due to insane prices. The Seekers are "good, but really not Hasbro" quality and, yes, are prone to breakages and misassembly. P/CP is a well done KO all in all, but doesn't fit together 100$ perfectly in vehicle mode and his head keeps wanting to come off if I movie it.
City Commander's head (KO or original)
Henkei Hot Rod's bio card
Universe Ironhide / Ratchet's gun/blade attachment