City Commander's (and its various repaints) proportions were soooo off-putting; I thought it would look better in person, but nope. I bought Henkei Prime and official DIA Commander, only to give both away to my nephew one month later. What a let-down.
MT Hypernovae was a super let-down for me too. Once the "ermahgerd teh wingz" feeling subsided, it quickly dawned on me that I spent almost $200 (tfsource) on a glorified deluxe. The toy was a lazy retool and the asking price was ridiculous, so I sold it to a member a few months later at a price I felt was fair: $80(-ish) shipped.
TW Trace's transformation was cumbersome and the plastic felt "brittle" so I sold it a month or so after purchasing it as well. I also snapped off a piece of plastic during the first transformation, which quickly soured my feelings for Trace.
PE Warden. Nice robot mode...aaaaand that's it. Transformation to the "tank" mode was lazy and quite the afterthought. Warden posed nicely, but left me desiring more, so I sold him three months later for $70 or so.
FWI-Jetfire upgrade. Wow, for $100 the whole upgrade felt like cheap model-grade plastic. I was so afraid to move the parts so I quickly sold it for $60 a month later.
KFC Surgeon. Despite the negative feedback on this mold, I still bought it for $40 new. Upon receiving it, I got so upset that a few hours later I took a hammer to the figure. True story...
MT Battle Tanker. SUPER LAZY USE OF THE TANKER ADD-ON!!!!!!!!!!!!!!!!!!!! Heavy parts meant posing Optimus was nearly impossible. I sold Battle Tanker for $20 a few weeks afterwards.
FP Last Chance. I wanted to see what all the initial hype was about. Upon seeing it in person, my response was: "I spent $60 on THIS?!?!" Sold it for $10. Didn't care to try and spend towards M3.
I sold many others as my collection needs changed, but the above toys were utter letdowns for me.