Wow... SO many toys to choose from! Let's see...
RiD: Ultra Magnus. Ridiculous bot mode, really just an afterthought so he could be combiner parts for Omega Prime.
Armada: Can I only pick one?? After the fantastic complexity and originality of RiD, this whole line was a disappointment. Sideswipe is an epically bad figure, as is Prime. I'm going to say Prime was the worst because his super mode was literally a brick. Who molds legs in place anymore?? That's like back in the day when our He-Man figures would "ride" their mounts by plugging into a hole on their back and the mount's saddle would have fake rider legs molded onto it!
Energon: Bots definitely got screwed in this line. Fatty Prime takes it, though. I would also mention KB Toys Demolishor, since it was a 2nd repaint of a lame Armada figure, and further confused by having the same character in the main toy line with an entirely new toy to match his look on the show.
Cybertron: I can't name names because it was too awful to watch; I caught the original Japanese version online instead. I think Supreme Starscream has to be the worst though. Unicron and Primus should be the only Transformers offered at this scale. Just look at Supreme Cheetor, ROTF Devastator, or Ultimate Bumblebee. Which leads me to...
TF2007: Ultimate Bumblebee. 6 times the size of the deluxe, and for what?? Lights and sounds??
Classics: Skipping recolors, I'd say Hasbro's voyager Megatron or the deluze 2-pack Prime v Megatron. Voyager Megs was a fine toy, but I don't understand the Nerf colors. Takara's version looks fantastic. As for the 2-pack, Prime's punch gimmick messes w/ articulation and his design is a little wonky. Same goes for tank Megs in this pack. Overall though, this line was AMAZING.
Animated: Voyager Grimlock. Snarl sucks (flaming Cheeto, anyone??), but Grimlock should have been so much better than he was. That chest gap in dino mode is unforgivable.
Universe: Hot Shot. The figure was a good step forward from the original, but he took up a valuable spot in a lineup that could've gone to a character far more deserving of an update - so while the toy itself isn't a disappointment, Hasbro's decision to green light him instead of another character is.
Classics 2.0: Ironhide, Ironhide, Ironhide... So much potential, so terrible as a figure. From his smurf face down to his tangram, gap-riddled alt mode, this figure is just awful. Try getting him to look straight ahead instead of down at the ground. It's not possible. They could've gone way simpler on this figure and had a total win, but they overshot the mark and made a red and blue plastic turd instead.
ROTF: Scalpel. Nice idea, terrible execution. Jolt could be in the running too. Awful bot mode.
Generations: Thundercracker - cuz he's not out, and probably never will be. Really, Hasbro?? You're gonna give us 5 of the 6 seekers and not the last one??