Wow! I'm incredibly overwhelmed by all the positive response and by the newsing! Thank you everyone. Although some of these entries have been a lot of work to write I've appreciated the support all the way.
I've traded in my old crappy crashy PC for a shiny new Mac (I expect my polo neck and beret to spontaneously appear at any second) so hopefully nothing will get in the way of the last four entries.
To celebrate, let's have some honourable mentions for those who didn't make the list:

"Open, dammit! Open!"
There's nothing like playing with a whole planet to make you feel like God. On the other hand there are only two planetformers (unless you count Darth Vader), they're horribly out of scale to everyone else and they make my Micromasters feel inadequate.

"Nyeeeeah, what's up, Blot?"
On the one hand, a rabbit mode is perfect for a recon agent or a scout who needs to get behind enemy lines without being seen. On the other hand, he's a
bunny!
Lent his accent to Animated Blitzwing and never got it back.
Volkwagens would have been a better alt mode were they not a Bumblebee-only club.

"Now that's a well made shoe!"
Heh heh. Yeah. Good times.