rotavator

Big one?

I would want one that provide drive to the wheels, rather than just powering the tines (such as Merry Tiller).

Small one?

I have a Mantis, and that is great for tilling the beds in the spring, and keeping the soil open between my shrubs and herbaceous plants.  Time consuming for a large area, compared to a bigger machine, but the benefit of "having one" outweighs the hassle of hiring one and then it rains that day ...