Classics voyager Prime
It's a good mix of quality, detail, a good transformation, a recognizable character, and styling that represents Transformers. The size is big enough to be significant, yet small enough to fit comfortably on a desk without being intrusive. Plus, you can tell them that the original price was only about 30$, whereas with MP figures you might scare them with the 100-400$ price tags that go along with them.
If all else fails though, you could always ask them if they want to have a look at your Erector