There were too many cool Transformers to name one, but Megatron was so kick-ass of a toy that I think I will have to go with him. But in the comic-books/cartoon I always thought Fortress Maximus kicked ass!!
The Transformers was a concept that came from the Japanese Takara series of transformable robots. But the Transformers as we know them are purely an American creation. The story of autobots against decepticons was molded into a comic book first. Then the story became a 5-part mini-series. If you watch the first 5 episodes today, you will notice that the story arc ends with the decepticons defeated and the autobots ready to board a space ship bound for their home world of Cybertron. Because of the popularity, they decided to turn it into an ongoing series. Which is why they go from the autobots ready to go home in the mini-series to the autobots still stuck on earth, facing many adventures against the evil decepticons.
Although the cartoon story and characters were American, the animation was done by Japanese artists. The same goes for G.I. Joe. American tale brought to life by Japanese artists.
Optimus Prime was my favorite toy, but Starscream was my favorite character in the cartoon. He was such an arrogant jerk. And he was voiced by Chris Latta, how cool is that? If I had a time machine, 1994 would have never happened and he'd still be voicing cowardly cartoon villains.
They were never made entirely out of metal. Some of the original G1 (but not all) had metal parts, such as Sideswipe's metal lower legs and Optimus's chest. In the current generation, I believe the line known as the Alternators in the US (where they are made out of plastic) is all metal in Japan. However, metal does have a downside: the paint can chip very easily.
My favorite recent TF is Anniversary Optimus Prime, my favorite G1 toy is Perceptor (which I still have).