Probably RiD Omega Prime, though I've never compared him to Optimal Optimus who may be taller/more massive.
Brave Maximus. I've also got Cybertron Supreme Starscream and Victory Saber, who're probably the second and third largest of all the TFs ever made.
probably my palisades devastator staue though wether its an actual 'transformer' is open for debate. i think the fan-freaking-tastic custom omega sculpt-bot made thats standing next to him is slightly bigger but as a custom i dont know if it counts either. so i guess my biggest official transforming transformer is mp prime.
your awesome i went and looked at the metallic green LT friday....i need to pay off my charger before i seriously consider getting a camaro....how does it drive?