In your experience, which consulting firm offers the best internships?

I'm afraid there is not unique answer here. In a consultancy, your work will largely depend on the project and your manager. Ultimately, you are asking the wrong question: start with the type of work you actually want to do (more sales, technical, strategy, reorg...) and figure out which firms / offices do more of that work. Then, assuming you do manage to get more than one internship, select based on your findings. You can alway make your own wishes known at the onset, but that might also be a risky move if you sound too pushy.

Yes, some internships will yield you more learning than others - but it would be hard to know ahead of time. Sorry!