You can already tell which alternative I've chosen. :)
There is a sense in which Calvin is the guy that Presbyterians (indeed, anyone in the Reformed tradition, which encompasses a number of other Protestant denominations) view as the founder of their branch of theology. Even if Calvin didn't originate a particular doctrine himself (his work owes a great debt to Augustine, in particular), it is often Calvin's particular take on it (often encapsulated in the acronym, "TULIP", which stands for "Total Depravity," "Unconditional Election," "Limited Atonement," "Irresistible Grace," and "Perseverance of the Saints") that Presbyterians look to for authority.
At least, so it is often assumed, both by Presbyterians themselves as well as those outside of the Reformed tradition. In reality, it's generally not so clear-cut. Presbyterians, for example, often either don't know the tenets of "TULIP" (and I don't really blame them, as the acronym's letters each come from the adjective describing the doctrine, and opposed to the core word of the doctrine itself. Knowing "Total, Unconditional, Limited, Irresistible, and Perseverance" doesn't tell you as much as knowing what or who those adjectives refer to, even if you know those things, but don't know the adjectives.) or actively argue against some element of one of TULIP's "petals" (even a Presbyterian pastor might argue against how "limited" the atonement is, for example). Of course, the formulation "TULIP" itself was created in response to Arminian thought by some of Calvin's followers well after Calvin's death, but it nonetheless is considered to be a reliable short-form of Calvin's teachings. In any event, even fewer Presbyterians have an understanding of Calvin's Institutes of the Christian Religion, which detail his thoughts far more thoroughly, than they do of TULIP.
But even if Calvin's teachings are not universally agreed to (and I myself am pretty conflicted on that count), there is a sense in which he left a legacy to which not only Presbyterians are indebted, but indeed the United States and other western democracies in the secular world, as well. For one thing, our representative form of government--in particular, the system of checks and balances inherent therein--is an outgrowth of Calvin's teachings on human fallibility. Of course, Calvin's insistence upon reason and academic study is of great importance to those of us who also try to find an intersection between matters of faith and secular understanding. Montreat Conference Center is hosting a conference, starting today, celebrating Calvin's 500th birthday, specifically calling it "an opportunity to rediscover Calvin’s significant and sometimes misunderstood legacy." One of the professors I work for, Dr. John L. Thompson, will be giving a lecture as part of the conference tomorrow: "Psalms of Cursing and Lament as a Prism for Calvin's Use of Scripture." It's his first trip to Montreat. Wish I could join him.
UPDATE: July 15, 2009. This week's episode of The God Complex featured a discussion of John Calvin's legacy. They were nice enough to give me a shout-out at the end as "Intern of the Week," which means nothing more than that I gave them a theme concept for how they would read the names of the people involved in putting the show together at the end of the podcast. I'd invite you to have a listen, particularly for those credits at the end. Can you guess what theme I gave them? I'll bet you can! ;)