I can't name a single TF that's the wimpiest, so I'll list a few Autobots and Decepticons that I think are strong contenders for this embarassing designation.
AUTOBOTS
1) Bumblebee. He's competent in the 2007 movie as a soldier, but a complete lil bitch lol in the cartoon and comic. His only redeeming features are that he gets along with humans and has above average courage. Useless in a fight, though.
2) Perceptor. Technically I guess he had decent combat skills but when he was getting raped by Soundwave's minions in the movie it didn't vouch well for these supposed talents.
3) Rachet (comic version). He was like a cranky version of Bumblebee and made Megatron laugh when he shot at him.
4) First Aid. I still don't know what use he had. Rachet was a superior "doctor" and almost every other Autobot was a better warrior.
5) Cliffjumper. Although he was marginally stronger than Bumblebee, he was stupid and reckless. Bad combination.
6) Jazz (movie). I don't think Jazz did anything of note in the battle at the end. He did manage to grab onto Devastator but got thrown into a building and then shot and ripped in half by Megatron. Not the kind of things that he wanted to be remembered by I'm sure.
7) Wheelie. Arguably one of the most childish characters in the series, was weak and his main weapon was a SLINGSHOT. Yeah.
8 ) Silverbolt. The leader of the Aerialbots (JETS) was afraid of heights. Did I mention he was a JET in charge of a bunch of other JETS but was afraid of heights?
9) Ultra Fagnus... I mean Magnus. Physically he was supposed to be one of the more powerful Autobots but I just remember him getting b*tch slapped by Galvatron and company, which in turn almost cost the Autobots the war and the fight against Unicron.
DECEPTICONS.
1) Laserbeak. Had some sort of piss poor lasers mounted but physically was useless. The only purpose he served was to go scout out things that Soundwave was to lazy to get involved with.
2) Ravage. Somehow he scanned a cat of some sort and became a metallic version of one. Do you ever remember any Autobot being scared of Ravage? Did Ravage ever win a fight? I think his main job was to fetch newspapers for Megatron or chasing Laserbeak.
3) Frenzy (2007 movie version). Frenzy was sly and useful when it came to hacking into computer systems, but he was a pansy when it came to fighting. He had some little ratatattat guns and some flying disks but what affect would those have on a full-sized Autobot?
4) Bombshell. I guess he could inject a cerebro shell into another living being and control them, but he was fat and slow and nobody liked him. Megatron tolerated him just because he liked his useful skill.
5) Sharkticons. As a group they were pretty rough, but one-on-one they were pretty sad. All they had was teeth, but no guns and they were slow. Even Wheelie would have a chance.
6) Cheetor. I hated Beast Wars because they sucked and Cheetor was probably the gay mate for Ravage.