I would still recommend Goodbye, Columbus as it's a great work. There's quite a bit to it, even if it just scrapes past the hundred page mark. But I suppose Portnoy's Complaint, Sabbath's Theater, American Pastoral, The Human Stain, and Operation Shylock are his five big hitters. I'm leaving out The Ghost Writer, Zuckerman Unbound, The Anatomy Lesson, The Prague Orgy, and Exit Ghost since these are Zuckerman books and a series in themselves and you only want to read four or five.
The 1,001 Books To Read Before You Die book suggests the following six: The Plot Against America, The Breast, American Pastoral, Sabbath's Theater, Operation Shylock, Portnoy's Complaint. The Breast, however, is about seventy pages and is the first of the Kepesh trilogy, which means that if you remove it, it's the same five books I mentioned in the previous paragraph.