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I think you're asking one of two things. Actual service (who actually GETS the subpoena) should be made on statutory agent of the LLC. Each business entity is registered with the Arizona Secretary of State, and the AZ Corporation Commission has a searchable database that lists the statutory agents of each company. You will need to hire a process server to deliver the documents to that agent.
If your question is which of those two individuals you should depose, you can let them identify the most knowledgeable individual. Rule 30(b)(6) of the Arizona Rules of Civil Procedure lets you identify a specific topic to be discussed in a deposition. The recipient of a subpoena for a Rule 30(b)(6) deposition is then obligated to identify the best individual to discuss the subject matter.
Depositions are actual court proceedings with their own set of rules. You should have an experienced litigation attorney representing you so that the testimony you solicit in the deposition can be admitted later on at trial.
Disclaimer: This post is for informational purposes only, and is not intended to provide legal advice or establish an attorney/client relationship. Legal advice should obtained by discussing the specific facts of your case with an attorney licensed to practice in your area.
You don't say whether the accountant is an internal employee or an outside CPA firm...but presumably you meant 3rd party CPA (by referring to subpoena) even though saying "accountant at the LLC" could be deemed to imply an internal employee.
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.