Cars and Ratatouille on Blu-Ray are great showpieces. The Pirates of the Caribbean BRDs are good as well.

There's also a good list here for punishing bass: http://www.svsound.com/questions-faqs.cfm#moviedemos

Really depends what you love. Whatever it is, your speakers will improve it. IMO, don't buy movies you'll hate, tho, just because they have a few demo moments (*cough* Aeon Flux *cough*).