Ironhide's tailgate

 

Definitely Optimus' hood ornament insignia or Ironhide's embossed tailgate insignia.

That's another thing I enjoy about the movieverse designs. With a few obvious exceptions*, the vast majority of insignias are (relatively) hidden, especially in robot mode. It's not just a 24 inch Autobot insignia slapped onto Optimus' shoulder or a three foot wide 'Con insignia front-and-center on someone's chest. It's a tiny little insignia embossed/coloured onto Bumblebee's/Starscream's/Jazz's headcrests, or a smaller Autobot insignia on Optimus' chest and the like.

*Mixmaster's cement drum insignias, Barricade's motto/insignia, etc..

And before people start complaining about the lack of "obvious" insignias on the designs, we all know that if the insignias were front-and-center and obvious, there'd be complaining about it and how "oh, it's bad enough that the bad guys look like monsters in order to differentiate between them, now we need Bay to shove the giant good-guy and bad-guy symbols in our face?!".  

 

Optimus first, Mix's 2nd.

 

mixmaster's

 

I'm very fond of Mixmaster's insignia, the little piece on his hood with this pointy Decepticon insignia head. I was quite amused the first time I saw it