I was in aircraft sales for about 2 years and I got quite a sampling of the GA market.
The one(s) that really stick out in my mind as rather unlikeable were the AC114/112.
Comfortable enough cabin, but the flying characteristics were uninspired. Not much power, not much speed, dull handling. And it existed in a market segment where the Bonanza and the 210 had it totally outclassed. No wonder the 2 aircraft we had generated little interest.
Some other ones that come to mind:
Twin Comanche- Duchess/Seminole performance with a tiny cabin. Landing perspective was terrible too.
C206- I can appreciate that this is a utilitarian airplane, but it lacked any kind of fun.
Navajo (not Chieftain)- Granted, I only flew AMF Hos, but they just didn't fly as well as I thought they should. The Chieftains seemed noticeably stabler and the extra power was nice too. Of course I flew them day in and out for over a year, so I might have just gotten used to them. But I do recall my first flights in the PA31 as being unimpressive. Again, AMF experience only.
CE525 (CJ1)- I spent a couple hundred hours in the bigger straight wing Citations (II and Ultra) and I just thought the CJ1 was awful. Horrifically tiny cockpit and I recall the airplane just being severely limited in a lot of ways.