LF for Optimus Prime Transformer toy. Which is the best one that suits at 5 year old?

Hey there. As the title states I'm looking for an Optimus Prime Transformer toy for my son who's turning 5 years old. There are many different ones out there, but which one do you think is the best bang for your buck, and would be suitable for a 5 year old. Not looking for something puny that's like 6" tall. Needs to be something larger and more impressive.

Thanks!