Trypticon

Trypticon was originally a robot dinosaur that transformed into a city. They've since made some versions where he's just a robot who transforms into a dinosaur, which is lame, but considering people mostly remember him as a robot dinosaur, it makes sense. Sadly the Kre-O Trypticon Kreon minifigure doesn't transform into a city, just a robot, but I'm slightly okay with that because tiny robot dinosaur.

The Kreons that have been released in anticipation of the Transformers 4 movie, what with the dinobots and such, have been pretty interesting. They actually get new sculpted parts to help in the transformation, and some of them actually look vaguely like the animals they're supposed to transform into. And even cooler, some of them actually use all the same pieces so the figures actually are transforming, not just putting on a new costume and being pushed onto their sides.