Obviously Grimlock for the Autobots. It would be ignorant as fuck not have him now. For a Decepticon they can have Assfuckshitturdotron.
Now that we have 'triple changers', in the tradition of spinal tap - lets turn it up 11 !!!
Six Changers !!!!
There are various Six Changers, as you can see in the link -
The most memorabe Six Changer for Japanese G1 is Ninja consultant Six Shot.
Some 'relative' six changers that could be brought in after TF3 could be other Japanese G1 characters like Sixknight (weaker brother?) and Greatshot (a rumored Sixshot 'reformatted').
A six-changer? Holy crap we already got 50+ bots in the 2nd movie. A big LOL
Wheeljack would be cool, and so would Mirage. Not sure how they would pull off the iinvisibility.
Hound and Shockwave FTW.
"I can change almost anything - But I can't change human nature." -Dr Manhattan/Jon Osterman