As of now I completely my Transmetals through online and thrift shopping. I found a complete Optimus for $5 at a second hand shop. $6 Rampage missing his Gun, Missles and two legs. I've since found them online and all 12 of these guys are complete now

I got the Autobot brothers Sideburn, Prowl and X-Brawn for Christmas when I was 11 and a couple years ago set out collecting the three combiners Rail Racer, Omega Prime and Landfill. Through online stores and sellers I've got them all complete minus Ultra Magnus' missles

These may not be the best figures or the rarest but they hold sentimental value to me and they showcase some unique aspects of Transformers history and engineering. These are the toys I'm most proud of owning.
What are you most proud of in your collection? What do you consider your "Holy Grail"?