First things first, you cannot disclose any client or firm confidential information to your prospective new employer. You probably knew that, but it's important enough to be worth stating.
Anything in a published application that you authored is "fair game" for use as a writing sample. Responses to Office Actions, specifications, claims, etc. All that information has been donated to the public, and so clearly raises no issue of confidentiality.
As for the appropriate writing sample to send, it will depend on what you are applying to do at the new firm. Are you applying for a prosecution slot? If so, send in one application that you think is the best representation of your work. Applying to be a litigator? Send in one brief or motion that you drafted, again, which is the best representation of your work. If the client is looking for a particular technical expertise, you might also want to submit a copy of a journal article or two that you may have written on the topic, or perhaps your dissertation if you have a graduate level degree in that technical area.
I would offer, but not provide, additional writing samples to the prospective employer for two reasons. First, you want the employer to see your best work product possible. You don't want them to be distracted by an error you might have committed in one of the many additional writing samples you might provide. Second, it is unlikely that the prospective employer will want to review a pluirality of writing samples for one candidate, nor is it likely that they have the time to do so. Pick your best work, submit it, and offer to provide additional samples if the prospective employer asks. And if they do ask, I would again limit your response to the best 1-2 additional samples, for the same reasons stated above.
Good luck!