Normal military tanks? Boring. How about a spider mecha, which you can use to cause havoc around the city of Chicago? Now that's what I'm talking about.
You can see the spider tank in action in this preview by Eurogamer:
The spider tank actually exists in a mini game where you get points for destruction, in case you were wondering how this fits into the actual game. The mech can climb any surface, and it comes equipped with machine guns and rockets. You can also use the spider's legs to destroy things, just like you'd be able to run over something with a normal tank. Heck, you can even upgrade the spider tank -- it comes with a skill tree and everything.
All in all, a very cool inclusion -- and I say this as someone who hates spiders.
Watch Dogs: Spider Tank - Eurogamer Preview[Eurogamer]