Except Rhea. When Cassini flew through the ring plane near Rhea - that is, through Rhea's orbit, above its equator - there was a drop-off in magnetic particles. Huh, the scientists said. This means there's probably a dim ring of uncharged particles around Rhea. That's pretty cool; let's take a picture the next time the orbiter goes by. So the orbiter went by again, they snapped pictures - no ring. Nothing. Just a gap in the charged particle density around Rhea.
Why? We don't know. Some kind of magnetic field around Rhea that repels the charged particles? Probably. Why and how did it get there? We don't know. But this mystery is enough to make Rhea - Saturn's second-largest moon - interesting, and earns her a spot in my top ten.