I replaced the right motor mount on the the slant six (the one you have to jack up the engine to reach, of course) twice. Some magnificant moron designed the mount with two flat plates that had the rubber merely bonded between them. When the engine revs up, the bond is placed in tension. It didn't take much to break that bond. After the first replacement mount broke, I drilled it's replacement for a couple of 1/4" bolts to go through both plates and loosely secured the bolts with nylocks. That limited the amount that the engine could stretch the rubber.
The traansmission mount on the tailshaft housing of the tranny also went bad (same reason; two plates with a piece of rubber bonded between them; I really wanted to meet that mute donkey engineer). Adding insult to injury, the mount had captive bolts that made it impossible to remove, even with the tail end of the tranny jacked into the floorboard, unless one removed the little crossmember it was mounted on. The catch to that was the little frames stubs that the x-member bolted to were also the anchors for the torsion bars for the front end. One had to back off the preload on the torsion bars before removing the x-member to avoid hopelessly warping the frame stubs (it was a unibody). I said the heck with all that and bungee corded the tailshaft housing down.
I swear, besides the hopelessly incompetent engineering, that the car was jinxed. It was always getting flats. I had a battery explode when cranking the engine one day (then almost got into a fight with the parts guy where I got the battery because he was convinced I had been jumping the battery). There was always something stupid going wrong with it. I was so happy to get rid of the suck, sorry sack of...you get the point.
The Pontiac was nothing but a glammed up Chevy (sorta like putting lipstick on a pig but not getting Miss Piggy). Anything stupid the Chevy engineers designed, the Tin Indian engineers made worse and any thing Chevy got right, Pontiac messed up. That one was another car I was happy to get rid of.
The engine in the Caddy had soft rings and started getting excessive blowby not too long after I got it. I couldn't drive it over 50 mph or it would blow oil all over the engine (fortunately this was back during double nickle so that wasn't too much of a problem). The front end started wearing out shortly before I got rid of it. Otherwise, it was a pretty nice car.
The others were all pretty good rides.
The E300 had a 240" six (danged good engine!) with a C4 behind it. That was a nice tranny. When I finally wore out the engine, I replaced it with a 302 and an AOD (Daddy helped). That was a sweet swap. I got more power and better gas mileage. Except for the throttle linkage, all the parts used were factory (from boneyards).