This is just a list based on toys i own or have owned at one point or another:

Alternators Nemesis Prime - Bonnet would never close properly and literal brick legs. I enjoyed the engine block/gun more than the main toy!

One of the Deluxe ROTF Bikes. Not even sure which one i have, blessed Mum got it for me for Christmas so i can't bring myself to get rid of it, but the robot is an abomination. At least i can leave it in bike mode!

07 Movie Deluxe Final Battle Jazz. Hated those arms! It's not that bad really, but was made to look a lot worse toys by being released near BB'08.

Superlink Deluxe Grand "Fattimus" Convoy. I'm semi-partial to Fattimus' robot mode (at least with delicious chrome) so he stays in my collection, but his truck mode is puke-worthy.

ROTF Deluxe Jolt. A decent enough alt mode but a poorly thought out robot and i hate his feet design.

Animated Blitzwing. Another that stays in Robot mode, considering he has almost no parts that actually connect and stay in any of his other modes.

Universe Ultra Powerglide. Nice headsculpt, not so much anything else.

07 Movie Deluxe Brawl. The one with the never-pegs-in shoulders.

Animated Cybertron Mode Voyager Megatron. At least it's the only time i've succumbed to one of Megatron's blob alt modes!

Galaxy Force Inch Up. Like a surfboard on monster truck wheels!