I don't think there's any legitimately great defenses in CFB this year. I thought Michigan might have been the closes to great, but OSU happened. Clemson gave up over 30 to Carolina, Bama got pushed around by Citadel(!) and most everybody else ranges from decent to putrid.
ND may be undefeated, but outside of Michigan they don't have that splashy win. They're a lot like Michigan where they have a good defense but a lackluster offense. They might be able to slow down high-powered offenses to an extent, but they don't have the O to keep up. IMHO, they'd get beaten soundly by Bama, Clemson, Georgia, and Ohio State (as long as the Bucks show up). I don't think they'd beat Oklahoma either.
This season's theme is offense, offense, and more offense. This might be the year where Alabama gives up 50 to an Oklahoma or Georgia, and still win.