They did the plastic versions in die-cast as well, the series was called Metal Heroes. Here's a link to the info:I think the best quality you're going to find in 1:64 scale are the ones done by Hasbro................but they are as much plastic as diecast. They did however get the detail and scale correct.
Oh yes, I forgot about that one. mr0072003 is definitely right with that one, it is a beautiful piece.What you are looking for is the HASBRO Titanium Optimus. It was released in 2007 and was part of the TITANIUM series. I have it, and when I opened it, was surprised it was all Die cast and ruber tires great detail.
Here is an eby link to what I am talking about