To me, the most amazing moment in Calvin's Institutes is when he says that the content of general revelation is (though we fail to perceive it) exactly the same as the content of special revelation
Theologically, I think the richest part of Calvin's Institutes is his explanation of the unity of the OT and NT. The greatest patristic authors struggled with this problem - Tertullian, Irenaeus, Origen, Augustine - but Calvin's treatment is profounder and more satisfying by far
Ethically, the richest part of the Institutes is Calvin's exposition of the Decalogue. Each prohibition implies a positive responsibility, and each individual responsibility implies a larger social ethic. Almost nothing in patristic or medieval moral teaching is as rich as this
Doctrinally, Calvin's most stunning moment is his account of Christ's threefold office: prophet, priest, king. This amounts to a massive synthesis of the OT as a revelation of the Messiah. It incorporates seemingly disparate patristic themes into a single comprehensive statement
Liturgically, Calvin's finest hour is his treatment of the concept of 'sacraments'. He gives a powerful explanation of the asymmetrical unity of word and thing (a unity in which word always predominates), and shows that this theory of signs has deep roots in OT covenantal thought
I've read & taught Calvin's Institutes many times over the years and I think those are the highlights. And they all have one thing in common: they all centre on an interpretation of the Old Testament.
In my view, that's the secret of Calvin's theological genius, that his thought has such deep roots in the OT. There is a Hebrew orientation to Calvin's thought that is quite pervasive once you notice it, and almost unique in the history of Christian thought.
To return to the example of Christ's threefold office: It's astounding to reflect that no previous Christian author had ever noticed that the concept of 'Christ' (anointed one) arises from three specific OT anointings, so that the OT supplies the structure of christology
Another example: it is astounding to discover the unimportance of the Ten Commandments in patristic moral and social teaching
Calvin is arguably the most non-Marcionite thinker in history. In fact, one sometimes feels that his later followers (e.g. Puritans in England and New England) were living too much in the world of the OT to the neglect of the NT. An amazing and admirable error, if it is an error
